RFR: 8200102: NativeLibraryTest.java fails intermittently, unloaded count is not same as expected

[Brent Christian]

Please review this fix for the intermittently-failing java/lang/ClassLoader/nativeLibrary/NativeLibraryTest.java.

The change replaces System.gc()+sleep() with the more robust gcAwait() mechanic used elsewhere in the test base, [as pointed out by Martin](https://bugs.openjdk.java.net/browse/JDK-8200102?focusedCommentId=14382648&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14382648)(thanks!).

The new version of the test passes 100 times out of 100 on the test farm.

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Commit messages:
 - Add gcAwait() mechanism

Changes: https://git.openjdk.java.net/jdk/pull/1630/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1630&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8200102
  Stats: 52 lines in 1 file changed: 41 ins; 5 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1630.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1630/head:pull/1630

PR: https://git.openjdk.java.net/jdk/pull/1630