Add GitHub action to test PR on coredev.

This is a gitHub Action for testing a package PR on buildout.coredev.
For now (May 2023) it is an experiment, but I think we need it.
Reasons:

1. The robot tests on Jenkins are flaky/unstable, which most likely is due to several robot jobs running parallel on one node.
2. Python 3.10 is not working with the 'Python Shining Pandas' plugin we use on Jenkins.  We had it running with a different script for a while, but these changes got lost.  Should be restorable, but let's try GHA.

One thing to watch out for, is that robot tests are not always reported as failures.
On the one hand there is config for that on Jenkins:
plone/jenkins.plone.org#297
On the other hand I see the same problem locally:
#3537

TODO: put this in a reusable workflow in plone/.github:
https://docs.github.com/en/actions/using-workflows/reusing-workflows
But first we should see if it actually works.

Aftwerwards, we could install it in all relevant Plone repos using https://github.com/asottile/all-repos
But nothing too hasty. :-)

.github/workflows/coredev.yml

@@ -0,0 +1,75 @@
+# This is a gitHub Action for testing a package PR on buildout.coredev.
+# For now (May 2023) it is an experiment, but I think we need it.
+# Reasons:
+# 1. The robot tests on Jenkins are flaky/unstable, which most likely is due
+#    to several robot jobs running parallel on one node.
+# 2. Python 3.10 is not working with the 'Python Shining Pandas' plugin we use
+#    on Jenkins.  We had it running with a different script for a while, but
+#    these changes got lost.  Should be restorable, but let's try GHA.
+#
+# One thing to watch out for, is that robot tests are not always reported as failures.
+# On the one hand there is config for that on Jenkins:
+# https://github.com/plone/jenkins.plone.org/issues/297
+# On the other hand I see the same problem locally:
+# https://github.com/plone/Products.CMFPlone/issues/3537
+
+# TODO: put this in a reusable workflow in plone/.github:
+# https://docs.github.com/en/actions/using-workflows/reusing-workflows
+
+name: Test PR branch on buildout.coredev.
+on: pull_request
+jobs:
+  build:
+    strategy:
+      fail-fast: false
+      matrix:
+        python-version: ["3.7", "3.8", "3.9", "3.10"]
+        test-command: ["bin/test --all -m test_robot"]
+        include:
+          - python-version: "3.10"
+            test-command: "bin/test"
+    runs-on: ubuntu-latest
+    steps:
+    - name: Ubuntu setup
+      run: |
+        # needed for CMFPlone testUnicodeSplitter test:
+        sudo locale-gen nl_NL@euro
+        sudo update-locale
+        # Needed for Products.PortalTransforms WordTransformsTest:
+        sudo apt-get install -y wv
+    - name: Set up Python ${{ matrix.python-version }}
+      uses: actions/setup-python@v3
+      with:
+        python-version: ${{ matrix.python-version }}
+      # cache: 'pip' would fail because we have no requirements.txt
+    - name: "Install Python dependencies (pip)"
+      uses: "py-actions/py-dependency-install@v3"
+      with:
+        path: "https://raw.githubusercontent.com/plone/buildout.coredev/6.0/requirements.txt"
+    - name: Cache eggs
+      uses: actions/cache@v3
+      with:
+        path: eggs
+        key: eggs-${{ matrix.python-version }}-${{ runner.os }}-
+        restore-keys: |
+          eggs-${{ matrix.python-version }}-
+    - name: Run buildout
+      env:
+        package_name: ${{ github.event.repository.name }}
+        clone_url: ${{ github.event.pull_request.head.repo.clone_url }}
+        branch: ${{ github.event.pull_request.head.ref }}
+      run: |
+        echo "Using environment: package_name=$package_name clone_url=$clone_url branch=$branch"
+        # Create empty buildout.cfg, otherwise buildout gets confused.
+        touch buildout.cfg
+        buildout buildout:extends=https://raw.githubusercontent.com/plone/buildout.coredev/6.0/buildout.cfg buildout:git-clone-depth=1 buildout:auto-checkout+=$package_name sources:$package_name="git $clone_url branch=$branch" install instance test
+    - name: Run tests
+      env:
+        ROBOT_BROWSER: headlesschrome
+      run: ${{ matrix.test-command }}
+    - name: Upload parts/tests directory for debugging
+      uses: actions/upload-artifact@v2
+      if: failure()
+      with:
+        name: plone-coredev-${{ matrix.python-version }}
+        path: parts/test/