Add checkTestsHaveRun task to check tests have run (#28350)

Extend JUnitSuite to make sure the test is included in the report.

After merging this, nothing would happen. We should then invoke
`checkTestsHaveRun` from the jenkins nightly builds. That should then
fail the build until we merge #28347 and switch the nightly builds
to run on JDK11.

akka-stream-tests/src/test/java-jdk9-only/akka/stream/javadsl/JavaFlowSupportCompileTest.java

@@ -10,7 +10,9 @@ import org.junit.Test;
 
 import java.util.concurrent.Flow;
 
-public class JavaFlowSupportCompileTest {
+import org.scalatest.junit.JUnitSuite;
+
+public class JavaFlowSupportCompileTest extends JUnitSuite {
   @Test
   public void shouldCompile() throws Exception {
     final Flow.Processor<String,String> processor = new Flow.Processor<String, String>() {