RFR 8231710: Add timeout asserting method to test library

[Julia Boes]

Hi,

This test-only RFR proposes a tool for timeout testing for the JDK test 
library. It runs a task in a separate thread and cancels the task if it 
doesn't complete within a given timeout. Any exception thrown by the 
task is propagated transparently.

TestNG doesn't currently provide this functionality but it seems like a 
handy little tool to have. The webrev includes several sample use cases.

Bug: https://bugs.openjdk.java.net/browse/JDK-8231710
Webrev: http://cr.openjdk.java.net/~jboes/webrevs/8231710/webrev.01/

Any feedback appreciated.

Regards,
Julia