test: support running functionaltests in parallel by directory (#37918)

Define a CMake target for every subdirectory of test/functional that
contains functional tests, and a functionaltest_parallel target that
depends on all those targets, allowing multiple test runners to run in
parallel.

On CI, use at most 2 parallel test runners, as using more may increase
system load and make tests unstable.

.github/workflows/test.yml

@@ -163,16 +163,29 @@ jobs:
           cmake --preset ci -D CMAKE_BUILD_TYPE=Debug -D CMAKE_INSTALL_PREFIX:PATH=$INSTALL_PREFIX ${{ matrix.build.flags }}
           cmake --build build
 
+      - if: ${{ matrix.test == 'unittest' }}
+        name: unittest
+        timeout-minutes: 20
+        run: cmake --build build --target unittest
+
+      - if: ${{ matrix.test == 'functionaltest' }}
+        name: functionaltest
+        timeout-minutes: 20
+        env:
+          # With parallel tests, use shorter timeout for a single group.
+          TEST_TIMEOUT: 600
+        run: |
+          set +e
+          cmake --build build --target functionaltest_parallel -j 2 -- -k 0
+          exit_code="$?"
+          cmake --build build --target functionaltest_summary
+          exit "$exit_code"
+
       - if: ${{ matrix.test == 'oldtest' }}
-        name: ${{ matrix.test }}
+        name: oldtest
         timeout-minutes: 20
         run: make -C test/old/testdir NVIM_PRG=$(realpath build)/bin/nvim
 
-      - if: ${{ matrix.test != 'oldtest' }}
-        name: ${{ matrix.test }}
-        timeout-minutes: 20
-        run: cmake --build build --target ${{ matrix.test }}
-
       - name: Install
         run: |
           cmake --install build

.github/workflows/test_windows.yml

@@ -54,7 +54,15 @@ jobs:
       - if: ${{ matrix.test == 'functional' }}
         name: functionaltest
         timeout-minutes: ${{ inputs.functionaltest_timeout }}
-        run: cmake --build build --target functionaltest
+        env:
+          # With parallel tests, use shorter timeout for a single group.
+          TEST_TIMEOUT: 600
+        run: |
+          $ErrorActionPreference = 'Continue'
+          cmake --build build --target functionaltest_parallel -j 2 -- -k 0
+          $exitCode = $LASTEXITCODE
+          cmake --build build --target functionaltest_summary
+          exit $exitCode
 
       - if: ${{ matrix.test == 'old' }}
         uses: msys2/setup-msys2@v2