SUREFIRE-1101 Surefire does not shutdown thread-pools programatically af...

...ter test run has finished.

The threads executing tests in parallel should be shutdown after the test run has finished.
We had a significant number of Threads exhausting the resources in the integration tests.
This will shutdown these threads after junit Computer exits.
Now the Computer waits until these threads are gone. The same happens after timeout for parallel execution is elapsed.