Long test killer doesn't work on Java 11

When a test takes > 5 minutes, RunningTestsRegistry takes a thread dump of the entire JVM and its child processes, then interrupts the test thread, and if the test still doesn't stop, it kills the test suite JVM.

With OpenJDK 11, the path to jstack is wrong, the thread dump step fails, and the interrupt/kill steps are skipped.

Note that on IBM JDK 8 jstack is missing completely, so we may want to write the thread stacktraces ourselves. The downside is that we would lose synchronization/lock/deadlock information.