Improve diagnostics for JUnit tests which crashed or timed out on Jenkins

Add test container memory events printing to check if Linux OOM killer was active
Fix thread dump printing on junit test timeout
Process.pid() API is available since JDK9

patch by Dmitry Konstantinov; reviewed by Michael Semb Wever for CASSANDRA-21172

Author: netudima

.jenkins/Jenkinsfile

@@ -433,6 +433,11 @@ def test(command, cell) {
             if (!cell.step.startsWith("microbench")) {
               junit testResults: "test/**/TEST-*.xml,test/**/cqlshlib*.xml,test/**/nosetests*.xml", testDataPublishers: [[$class: 'StabilityTestDataPublisher']]
             }
+            // check if we had Linux OOM killer active within the test container which could kill forked JUnit JVM processes
+             sh """
+                echo "docker memory/oomkiller debug:"
+                cat /sys/fs/cgroup/docker/memory.events || true
+              """
             sh """
                 find test/output -type f -name "*.xml" -print0 | xargs -0 -r -n1 -P"\$(nproc)" xz -f
                 echo "test result files compressed"; find test/output -type f -name "*.xml.xz" | wc -l

test/unit/org/apache/cassandra/JStackJUnitTask.java

@@ -20,7 +20,6 @@
 import java.io.BufferedReader;
 import java.io.IOException;
 import java.io.InputStreamReader;
-import java.lang.reflect.Field;
 
 import org.apache.tools.ant.BuildException;
 import org.apache.tools.ant.taskdefs.ExecuteWatchdog;
@@ -77,6 +76,7 @@ public synchronized void timeoutOccured(Watchdog w)
                 ProcessBuilder pb = new ProcessBuilder("jstack","-l", String.valueOf(pid));
                 try
                 {
+                    pb.redirectErrorStream(true);
                     Process p = pb.start();
                     try (BufferedReader br = new BufferedReader(new InputStreamReader(p.getInputStream())))
                     {
@@ -100,22 +100,15 @@ public synchronized void timeoutOccured(Watchdog w)
 
         private long getPid(Process process)
         {
-            if (process.getClass().getName().equals("java.lang.UNIXProcess"))
+            try
             {
-                try
-                {
-                    Field f = process.getClass().getDeclaredField("pid");
-                    f.setAccessible(true);
-                    long pid = f.getLong(process);
-                    f.setAccessible(false);
-                    return pid;
-                }
-                catch (IllegalAccessException | NoSuchFieldException e)
-                {
-                    System.err.println("Could not get PID");
-                }
+                return process.pid();
+            }
+            catch (UnsupportedOperationException e)
+            {
+                System.err.println("Could not get PID");
+                return -1;
             }
-            return -1;
         }
     }