Disable RTDB unit tests for now in XC15 CI (#12087)

.github/workflows/database.yml

@@ -31,8 +31,10 @@ jobs:
         include:
           - os: macos-12
             xcode: Xcode_14.2
+            tests: --test-specs=unit
           - os: macos-13
             xcode: Xcode_15.0.1
+            tests: --skip-tests
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v3
@@ -42,7 +44,7 @@ jobs:
     - name: Xcode
       run: sudo xcode-select -s /Applications/${{ matrix.xcode }}.app/Contents/Developer
     - name: Build and test
-      run: scripts/third_party/travis/retry.sh scripts/pod_lib_lint.rb FirebaseDatabase.podspec --test-specs=unit --platforms=${{ matrix.target }}
+      run: scripts/third_party/travis/retry.sh scripts/pod_lib_lint.rb FirebaseDatabase.podspec ${{ matrix.tests }} --platforms=${{ matrix.target }}
 
   integration:
     # Don't run on private repo unless it is a PR.