chore: add a CI target to run emulator integration tests using multiplexed sessions. (#3029)

Author: arpan14

.github/workflows/ci.yaml

@@ -36,6 +36,23 @@ jobs:
     - run: .kokoro/build.sh
       env:
         JOB_TYPE: test
+  units-with-multiplexed-session:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        java: [ 11, 17, 21 ]
+    steps:
+    - uses: actions/checkout@v4
+    - uses: actions/setup-java@v3
+      with:
+        distribution: temurin
+        java-version: ${{matrix.java}}
+    - run: java -version
+    - run: .kokoro/build.sh
+      env:
+        JOB_TYPE: test
+        GOOGLE_CLOUD_SPANNER_ENABLE_MULTIPLEXED_SESSIONS: true
   units-java8:
     # Building using Java 17 and run the tests with Java 8 runtime
     name: "units (8)"