output stack traces when unit tests time out

com.oracle.truffle.r.test/src/com/oracle/truffle/r/test/generate/FastRSession.java

@@ -207,6 +207,9 @@ public final class FastRSession implements RSession {
         try {
             if (!thread.await(longTimeout ? longTimeoutValue : timeoutValue)) {
                 consoleHandler.println("<timeout>");
+                for (StackTraceElement element : thread.getStackTrace()) {
+                    System.out.println(element);
+                }
                 thread.stop();
                 evalThread.ensureContextDestroyed();
                 evalThread = null;