Speed up SPM CI by reducing full Firebase rebuilds (#8234)

Author: paulb777

.github/workflows/spm.yml

@@ -48,10 +48,13 @@ jobs:
       matrix:
         target: [tvOS, macOS, catalyst]
         # Full set of Firebase-Package tests only run on iOS because of Analytics.
-        test: [objc-import-test, swift-test, version-test]
     steps:
     - uses: actions/checkout@v2
     - name: Initialize xcodebuild
       run: scripts/setup_spm_tests.sh
-    - name: Unit Tests
-      run: scripts/third_party/travis/retry.sh ./scripts/build.sh ${{ matrix.test }} ${{ matrix.target }} spm
+    - name: Objc Import Tests
+      run: scripts/third_party/travis/retry.sh ./scripts/build.sh objc-import-test ${{ matrix.target }} spm
+    - name: Swift Tests
+      run: scripts/third_party/travis/retry.sh ./scripts/build.sh swift-test ${{ matrix.target }} spm
+    - name: Version Tests
+      run: scripts/third_party/travis/retry.sh ./scripts/build.sh version-test ${{ matrix.target }} spm