JDK 9 RFR of JDK-8169971: tools/jlink/multireleasejar/JLinkMultiReleaseJarTest.java fails intermittently

[Amy Lu]

Please review this test-only change.

tools/jlink/multireleasejar/JLinkMultiReleaseJarTest.java

This test fails intermittently on Windows platform in the final cleanup 
step, @AfterClass, in which test tries to delete all dirs and files it 
created. Though it’s okay to leave this cleanup work to jtreg, I still 
make it done in test by making test to create everything under 
./testoutput (testdir), and delete this testdir directly with 
FileUtils.deleteFileTreeWithRetry(testdir).

Moreover, in FileUtils, as it tries to delete path with retry, we might 
want to give it a chance to "retry" in case of DirectoryNotEmptyException.

bug: https://bugs.openjdk.java.net/browse/JDK-8169971
webrev: http://cr.openjdk.java.net/~amlu/8169971/webrev.00/

Thanks,
Amy