CI: micro-optimize test collection & pass nox's --no-install

Pytest can be pretty slow to collect pip's entire test suite and
prepare for test execution. I've observed a ~15s delay from invoking
pytest to the first test running in CI in the worst case. This can be
improved by reducing how many files pytest has to process while
collecting tests. In short, passing tests/unit is faster than -m unit.

In addition, use nox's --no-install flag to skip redundant build and
install steps on the 2nd nox session invocation (for the integration
tests), which was made possible by the previous commit.

.github/workflows/ci.yml

@@ -149,17 +149,17 @@ jobs:
       - name: Run unit tests
         run: >-
           nox -s test-${{ matrix.python.key || matrix.python }} --
-          -m unit
+          tests/unit
           --verbose --numprocesses auto --showlocals
       - name: Run integration tests
         run: >-
-          nox -s test-${{ matrix.python.key || matrix.python }} --
-          -m integration
+          nox -s test-${{ matrix.python.key || matrix.python }} --no-install --
+          tests/functional
           --verbose --numprocesses auto --showlocals
           --durations=5
 
   tests-windows:
-    name: tests / ${{ matrix.python }} / ${{ matrix.os }} / ${{ matrix.group }}
+    name: tests / ${{ matrix.python }} / ${{ matrix.os }} / ${{ matrix.group.number }}
     runs-on: ${{ matrix.os }}-latest
 
     needs: [packaging, determine-changes]
@@ -180,7 +180,9 @@ jobs:
           # - "3.11"
           # - "3.12"
           - "3.13"
-        group: [1, 2]
+        group:
+          - { number: 1, pytest-filter: "not test_install" }
+          - { number: 1, pytest-filter: "test_install" }
 
     steps:
       - uses: actions/checkout@v4
@@ -198,29 +200,19 @@ jobs:
           TEMP: "C:\\Temp"
 
       # Main check
-      - name: Run unit tests
-        if: matrix.group == 1
+      - name: Run unit tests (group 1)
+        if: matrix.group.number == 1
         run: >-
           nox -s test-${{ matrix.python }} --
-          -m unit
+          tests/unit
           --verbose --numprocesses auto --showlocals
         env:
           TEMP: "C:\\Temp"
 
-      - name: Run integration tests (group 1)
-        if: matrix.group == 1
+      - name: Run integration tests (group ${{ matrix.group.number }})
         run: >-
-          nox -s test-${{ matrix.python }} --
-          -m integration -k "not test_install"
-          --verbose --numprocesses auto --showlocals
-        env:
-          TEMP: "C:\\Temp"
-
-      - name: Run integration tests (group 2)
-        if: matrix.group == 2
-        run: >-
-          nox -s test-${{ matrix.python }} --
-          -m integration -k "test_install"
+          nox -s test-${{ matrix.python }} --no-install --
+          tests/functional -k "${{ matrix.group.pytest-filter }}"
           --verbose --numprocesses auto --showlocals
         env:
           TEMP: "C:\\Temp"
@@ -251,7 +243,7 @@ jobs:
       - name: Run integration tests
         run: >-
           nox -s test-3.10 --
-          -m integration
+          tests/functional
           --verbose --numprocesses auto --showlocals
           --durations=5
           --use-zipapp