perf(.github/workflows): parallelize legacylibrarian tests (#4024)

Split the legacylibrarian tests into a separate job to improve CI time
by ~2min.

Author: julieqiu

.github/workflows/librarian.yaml

@@ -17,7 +17,25 @@ permissions:
   contents: read
   issues: write
 jobs:
-  test:
+  go-generate:
+    runs-on: ubuntu-24.04
+    steps:
+      - uses: actions/checkout@v5
+      - uses: actions/setup-go@v6
+        with:
+          go-version-file: "go.mod"
+      - uses: ./.github/actions/install-protoc
+      - name: Install Go tools
+        run: go install tool
+      - name: Check go generate
+        run: |
+          go generate ./...
+          if [ -n "$(git status --porcelain)" ]; then
+            echo "go generate produced diffs:"
+            git diff
+            exit 1
+          fi
+  legacylibrarian:
     runs-on: ubuntu-24.04
     steps:
       - uses: actions/checkout@v5
@@ -26,33 +44,29 @@ jobs:
           go-version-file: "go.mod"
       - name: Run tests
         run: |
-          go test -race -coverprofile=coverage.out -covermode=atomic \
-            $(go list ./... | grep -v -E 'internal/librarian/(python|rust|dart)|internal/sidekick')
+          go test -race -coverprofile=coverage.out -covermode=atomic ./internal/legacylibrarian/...
       - name: Upload coverage report
         uses: codecov/codecov-action@v5
         with:
           token: ${{ secrets.CODECOV_TOKEN }}
-  go-generate:
+  test:
     runs-on: ubuntu-24.04
     steps:
       - uses: actions/checkout@v5
       - uses: actions/setup-go@v6
         with:
           go-version-file: "go.mod"
-      - uses: ./.github/actions/install-protoc
-      - name: Install Go tools
-        run: go install tool
-      - name: Check go generate
+      - name: Run tests
         run: |
-          go generate ./...
-          if [ -n "$(git status --porcelain)" ]; then
-            echo "go generate produced diffs:"
-            git diff
-            exit 1
-          fi
+          go test -race -coverprofile=coverage.out -covermode=atomic \
+            $(go list ./... | grep -v -E 'internal/librarian/(python|rust|dart)|internal/sidekick|internal/legacylibrarian')
+      - name: Upload coverage report
+        uses: codecov/codecov-action@v5
+        with:
+          token: ${{ secrets.CODECOV_TOKEN }}
   create-issue-on-failure:
     runs-on: ubuntu-24.04
-    needs: [test, go-generate]
+    needs: [go-generate, legacylibrarian, test]
     if: ${{ always() && contains(needs.*.result, 'failure') && github.event_name == 'push' && github.ref == 'refs/heads/main' }}
     steps:
       - name: Create an issue for push event to main