CI: switch to GitHub Actions - step 3: test stage

This commit:
* Adds a GH Actions workflow for the CI checks which were previously run on Travis in the `test` stage.
* Removes the, now redundant, `.travis.yml` configuration.
* Updates the `.gitattributes` file.

Notes:
1. Previously, this "stage" would run on all `pull requests` events.
    The current set-up still does so, with one addition: pushes to `master` (merges) will now also use this workflow instead of the quicktest.
    This replaces the full run on tagging a release, meaning that things will essentially be the same.
2. As there are a couple of jobs which are "allowed to fail" (`experimental` = true), the build status may unfortunately show as "failed", even though all non-experimental jobs have succeeded.
     This is a known issue in GHA: https://github.com/actions/toolkit/issues/399

.gitattributes

@@ -9,7 +9,6 @@
 /.gitignore export-ignore
 /.phpcs.xml export-ignore
 /.phpcs.xml.dist export-ignore
-/.travis.yml export-ignore
 /phpcs.xml export-ignore
 /phpcs.xml.dist export-ignore
 /phpunit.xml export-ignore

.github/workflows/test.yml

@@ -0,0 +1,133 @@
+name: Test
+
+on:
+  # Run on pushes to `master` and on all pull requests.
+  push:
+    branches:
+      - master
+    paths-ignore:
+      - '**.md'
+  pull_request:
+  # Allow manually triggering the workflow.
+  workflow_dispatch:
+
+jobs:
+  #### TEST STAGE ####
+  test:
+    runs-on: ubuntu-latest
+
+    strategy:
+      # Keys:
+      # - experimental: Whether the build is "allowed to fail".
+      matrix:
+        # The GHA matrix works different from Travis.
+        # You can define jobs here and then augment them with extra variables in `include`,
+        # as well as add extra jobs in `include`.
+        # @link https://docs.github.com/en/free-pro-team@latest/actions/reference/workflow-syntax-for-github-actions#jobsjob_idstrategymatrix
+        #
+        # Note: while WPCS 3.0.0 is under development, the matrix will use `dev-master`.
+        # Once it has been released and YoastCS has been made compatible, the matrix should switch (back)
+        # WPCS `dev-master` to `dev-develop`.
+        php: ['5.4', '5.5', '5.6', '7.0', '7.1', '7.2', '7.3', '7.4', '8.0']
+        phpcs_version: ['3.6.0', 'dev-master']
+        wpcs_version: ['2.3.0', 'dev-master']
+        experimental: [false]
+
+        include:
+          # Experimental builds. These are allowed to fail.
+
+          # PHP nightly
+          - php: '8.1'
+            phpcs_version: 'dev-master'
+            wpcs_version: 'dev-master'
+            experimental: true
+          # Test against WPCS unstable. Re-enable when WPCS is not in dev for the next major.
+          #- php: '8.0'
+          #  phpcs_version: 'dev-master'
+          #  wpcs_version: 'dev-develop'
+          #  experimental: true
+
+          # Test against the next major of PHPCS. Temporarily disabled due to upstream bugs.
+          #- php: '7.4'
+          #  phpcs_version: '4.0.x-dev'
+          #  wpcs_version: 'dev-develop'
+          #  experimental: true
+
+    name: "Test${{ matrix.phpcs_version == 'dev-master' && matrix.wpcs_version == 'dev-master' && ' + Lint' || '' }}: PHP ${{ matrix.php }} - PHPCS ${{ matrix.phpcs_version }} - WPCS ${{ matrix.wpcs_version }}"
+
+    continue-on-error: ${{ matrix.experimental }}
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+
+      # On stable PHPCS versions, allow for PHP deprecation notices.
+      # Unit tests don't need to fail on those for stable releases where those issues won't get fixed anymore.
+      - name: Setup ini config
+        id: set_ini
+        run: |
+          if [[ "${{ matrix.phpcs_version }}" != "dev-master" && "${{ matrix.phpcs_version }}" != "4.0.x-dev" ]]; then
+            echo '::set-output name=PHP_INI::error_reporting=E_ALL & ~E_DEPRECATED'
+          else
+            echo '::set-output name=PHP_INI::error_reporting=E_ALL'
+          fi
+
+      - name: Install PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php }}
+          ini-values: ${{ steps.set_ini.outputs.PHP_INI }}
+          coverage: none
+          tools: cs2pr
+        env:
+          # Token is needed for the PHPCS 4.x run against source.
+          COMPOSER_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+
+      # Set Composer up to download only PHPCS from source for PHPCS 4.x.
+      # The source is needed to get the base testcase from PHPCS.
+      # All other jobs can use `auto`, which is Composer's default value.
+      - name: 'Composer: conditionally prefer source for PHPCS'
+        if: ${{ startsWith( matrix.phpcs_version, '4' ) }}
+        run: composer config preferred-install.squizlabs/php_codesniffer source
+
+      - name: 'Composer: adjust dependencies'
+        run: |
+          # Set the PHPCS version to test against.
+          composer require --no-update --no-scripts squizlabs/php_codesniffer:"${{ matrix.phpcs_version }}"
+          # Set the WPCS version to test against.
+          composer require --no-update --no-scripts wp-coding-standards/wpcs:"${{ matrix.wpcs_version }}"
+
+      - name: 'Composer: conditionally remove PHPCSDevtools'
+        if: ${{ startsWith( matrix.phpcs_version, '4' ) }}
+        # Remove devtools as it will not (yet) install on PHPCS 4.x.
+        run: composer remove --no-update --dev phpcsstandards/phpcsdevtools
+
+      # Install dependencies and handle caching in one go.
+      # @link https://github.com/marketplace/actions/install-composer-dependencies
+      - name: Install Composer dependencies - normal
+        if: ${{ startsWith( matrix.php, '8' ) == false }}
+        uses: ramsey/composer-install@v1
+
+      # For the PHP 8/"nightly", we need to install with ignore platform reqs as we're still using PHPUnit 7.
+      - name: Install Composer dependencies - with ignore platform
+        if: ${{ startsWith( matrix.php, '8' ) }}
+        uses: ramsey/composer-install@v1
+        with:
+          composer-options: --ignore-platform-reqs
+
+      - name: Verify installed standards
+        run: vendor/bin/phpcs -i
+
+      - name: Lint against parse errors
+        if: ${{ matrix.phpcs_version == 'dev-master' && matrix.wpcs_version == 'dev-master' }}
+        run: composer lint -- --checkstyle | cs2pr
+
+      - name: Run the unit tests - PHP 5.4 - 8.0
+        if: ${{ matrix.php != '8.1' }}
+        run: composer test
+
+      - name: Run the unit tests - PHP 8.1
+        if: ${{ matrix.php == '8.1' }}
+        run: composer test -- --no-configuration --dont-report-useless-tests
+        env:
+          PHPCS_IGNORE_TESTS: 'PHPCompatibility,WordPress'