JDK 10 RFR of test-only bug 8183320, et. al.: Better cleanup of temp files created by [N]IO tests

[Brian Burkhalter]

Please review at your convenience this patch for [1, 2, 3, 4]:

http://cr.openjdk.java.net/~bpb/8183320/webrev.00/

Note that without the existence check at line 60, the SetDefaultProvider test fails with a FileAlreadyExistsException.

Thanks,

Brian

[1] https://bugs.openjdk.java.net/browse/JDK-8183320
[2] https://bugs.openjdk.java.net/browse/JDK-8183321
[3] https://bugs.openjdk.java.net/browse/JDK-8183343
[4] https://bugs.openjdk.java.net/browse/JDK-8183344