RFR 8057150: Add more diagnostics to JMXStartStopTest

[Jaroslav Bachorik]

Please, review this trivial test change

Issue : https://bugs.openjdk.java.net/browse/JDK-8057150
Webrev: http://cr.openjdk.java.net/~jbachorik/8057150/webrev.00

This change is to provide us with more info about why the test fails as 
described in https://bugs.openjdk.java.net/browse/JDK-8057149

It seems that the string matching fails for some reason but currently 
available data in logs don't provide any clue. Hopefully, by adding more 
logging it would be possible to identify the problem.


Thanks,

-JB-