Speed up and simplify the configuration of testing CI. (#335)

This is primarily using the configuration matrix facilities of GitHub
actions to consolidate the overall configuration. Beyond avoiding
duplication, this also allows the default and release builds to run in
parallel. The checkout time is duplicated between these, but the rest of
the time is nicely parallelized. This has the most dramatic effect on
macOS builds. Overall, this should reduce the latency on testing from
22-28 minutes to 15-20 minutes from what I've seen which seems
worthwhile.

This does reduce the detail provided in the names of the different
configurations. However, the Bazel build mode is preserved. That seems
like the most critical pieces of information.

Much of this started with me just trying to learn more about GitHub
actions, but once understanding how the job matrix worked, it seemed
worthwhile to send out as an actual change.

.github/workflows/tests.yaml

@@ -10,82 +10,51 @@ on:
   pull_request:
 
 jobs:
-  macOS:
-    name: macOS
-    runs-on: macOS-latest
+  test:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest]
+        build_mode: [fastbuild, opt]
+    runs-on: ${{ matrix.os }}
     steps:
-      - uses: actions/setup-go@v2
-      - uses: actions/setup-python@v2
-        with:
-          python-version: '3.8.5'
-      - name: Install More Tools
-        run: |
-          go get github.com/bazelbuild/bazelisk
-          pip install gql PyGitHub
       - name: Checkout
         uses: actions/checkout@v2
         with:
           submodules: true
-      - name: Tool Versions
-        run: |
-          $(go env GOPATH)/bin/bazelisk --version
-          python --version
-          echo gql "$(pip show gql | grep Version)"
-          echo PyGitHub "$(pip show PyGitHub | grep Version)"
-      - name: >-
-          Build  (-c fastbuild: debug / no symbols)
-        run: |
-          $(go env GOPATH)/bin/bazelisk build \
-            --verbose_failures //...:all
-      - name: >-
-          Test (-c fastbuild: debug / no symbols)
-        run: |
-          $(go env GOPATH)/bin/bazelisk test --test_output errors \
-            --verbose_failures //...:all
-      - name: >-
-          Build (-c opt: release)
-        run: |
-          $(go env GOPATH)/bin/bazelisk build \
-            --verbose_failures -c opt //...:all
-      - name: >-
-          Test (-c opt: release)
-        run: |
-          $(go env GOPATH)/bin/bazelisk test --test_output errors \
-            --verbose_failures -c opt //...:all
 
-  linux:
-    name: Linux x86_64
-    runs-on: ubuntu-latest
-    steps:
+      # Setup Python and related tools.
       - uses: actions/setup-python@v2
         with:
           python-version: '3.8.5'
-      - name: Install More Tools
+      - name: Install python modules
         run: |
           pip install gql PyGitHub
-      - name: Checkout
-        uses: actions/checkout@v2
-        with:
-          submodules: recursive
-      - name: Tool Versions
+
+      # On macOS we need Go and to use it to install Bazelisk.
+      - uses: actions/setup-go@v2
+        if: matrix.os == 'macos-latest'
+      - name: Install bazelisk
+        if: matrix.os == 'macos-latest'
+        run: |
+          go get github.com/bazelbuild/bazelisk
+          echo "$(go env GOPATH)/bin" >> $GITHUB_PATH
+
+      # Print the various tool versions to help in debugging.
+      - name: Print tool versions
         run: |
           bazelisk --version
           python --version
           echo gql "$(pip show gql | grep Version)"
           echo PyGitHub "$(pip show PyGitHub | grep Version)"
-      - name: >-
-          Build  (-c fastbuild: debug / no symbols)
-        run: |
-          bazelisk build --verbose_failures //...:all
-      - name: >-
-          Test (-c fastbuild: debug/no symbols)
-        run: |
-          bazelisk test --test_output errors --verbose_failures //...:all
-      - name: >-
-          Build (-c opt: release)
+
+      # Build all targets first to isolate build failures.
+      - name: Build (${{ matrix.build_mode }})
         run: |
-          bazelisk build --verbose_failures -c opt //...:all
-      - name: >-
-          Test (-c opt: release)
+          bazelisk build -c ${{ matrix.build_mode }} --verbose_failures \
+            //...:all
+
+      # Run all test targets.
+      - name: Test (${{ matrix.build_mode }})
         run: |
-          bazelisk test --test_output errors --verbose_failures -c opt //...:all
+          bazelisk test -c ${{ matrix.build_mode }} --test_output errors \
+            --verbose_failures //...:all