jmx-dev RFR 8048005: Add test timing information to JMXStartStopTest

[Jaroslav Bachorik]

Please, review this simple change to add more debug information to the 
intermittently timing out test.

Issue:  https://bugs.openjdk.java.net/browse/JDK-8048005
Webrev: http://cr.openjdk.java.net/~jbachorik/8048005/webrev.00

Might be useful to see which particular test case gets stuck and takes 
the biggest amount of time.

Thanks,

-JB-