RFR (XS): 8199874: [TESTBUG] runtime/Thread/ThreadPriorities.java fails with "expected 0 to equal 10"

[David Holmes]

webrev: http://cr.openjdk.java.net/~dholmes/8199874/webrev/
bug: https://bugs.openjdk.java.net/browse/JDK-8199874

This test fails rarely and we can't determine why, so this fix simply 
adds extra diagnostic and reliability improvements:

1. Run in othervm mode to avoid capturing threads created by other tests 
in agentvm mode

2. Change the line-separator for pattern matching from \\n+ to \\R

3. Dump all the output if a failure is detected

Thanks,
David