RFR [12] 8211325: test/jdk/java/net/Socket/LingerTest.java fails with cleaning up

[Chris Hegarty]

Similar to the recent test fix for 8211092 [1], this change proposes to
modify the test so that it's main method does not return until all
threads, created by the test, terminate. This is important when run in
samevm / agentvm mode.

The change avoids any potential issues where jtreg may attempt to "clean
-up" these threads ( the clean up fails quite often as the closer thread
can linger in blocking close ).

http://cr.openjdk.java.net/~chegar/8211325.00/index.html

-Chris.

P.S. I'll fix some of the unrelated indentation issue before pushing,
which have been deliberately omitted to avoid confusing the actual
changes.

[1] https://bugs.openjdk.java.net/browse/JDK-8211092