RFR 8060120: Improve diagnostic output of StartManagementAgent test

[Jaroslav Bachorik]

Please, review this simple patch to add some more diagnostic information 
for com/sun/tools/attach/StartManagementAgent.java

Issue : https://bugs.openjdk.java.net/browse/JDK-8060120
Webrev: http://cr.openjdk.java.net/~jbachorik/8060120/webrev.00
This change is adding more details about what the test is actually 
performing. This is needed in order to get more insight into 
intermittent failures.

Thanks,

-JB-