RFR 8077402: JMXStartStopTest fails intermittently on slow hosts

[Jaroslav Bachorik]

Please, review the following test change

Issue : https://bugs.openjdk.java.net/browse/JDK-8077402
Webrev: http://cr.openjdk.java.net/~jbachorik/8077402/webrev.00

Currently, the test waits for 5*{timeout.factor} seconds for the target 
application to start up before timing out. On extremely slow machines 
even the timeout.factor will not assure a successful target application 
startup and the test will fail sporadically.

The proposed solution is to defer timing-out to the test harness. The 
test library will still take care of collecting the thread dump and 
terminating any left-over processes (ProcessTools.startProcess() 
methods). While this will not remedy the failures on extremely slow 
machines it is better not to introduce any additional arbitrary time-out 
limits.

As a part of this patch there is a small clarification in the javadoc of 
ProcessTools.startProcess() method and a null check in 
JMXStartStopTest$TestAppRun to prevent NPE hiding the timeout message.

Thanks,

-JB-