test: add job for running multiplexed session test using Java 8 runtime (#3062)

Author: rahul2393

.github/workflows/ci.yaml

@@ -72,6 +72,26 @@ jobs:
     - run: .kokoro/build.sh
       env:
         JOB_TYPE: test
+  units-with-multiplexed-session8:
+    # Building using Java 17 and run the tests with Java 8 runtime
+    name: "units-with-multiplexed-session (8)"
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-java@v3
+        with:
+          java-version: 8
+          distribution: temurin
+      - run: echo "SUREFIRE_JVM_OPT=-Djvm=${JAVA_HOME}/bin/java" >> $GITHUB_ENV
+        shell: bash
+      - uses: actions/setup-java@v3
+        with:
+          java-version: 17
+          distribution: temurin
+      - run: .kokoro/build.sh
+        env:
+          JOB_TYPE: test
+          GOOGLE_CLOUD_SPANNER_ENABLE_MULTIPLEXED_SESSIONS: true
   windows:
     runs-on: windows-latest
     steps: