[jdk16] RFR: 8258007: Add instrumentation to NativeLibraryTest

[Brent Christian]

This change adds some extra test output for NativeLibraryTest, primarily via an update to the ForceGC utility class.

It was observed that there was nothing preventing the Cleaner from cleaning the short-lived Object that ForceGC registers before await()/doit()/System.gc() is even called.

The new 'o' reference is kept alive until FoceGC.await() has been called.

We should find out a little more the next time NativeLibraryTest fails (or perhaps it won't fail anymore!)

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Commit messages:
 - Don't keep ForceGC'ing until we see the expected count
 - Add comment about new Object reference in ForceGC
 - fix spacing
 - Keep ForceGC'ing until we see the expected count
 - Add strongly-referenced Object to prevent cleaning before await() is called

Changes: https://git.openjdk.java.net/jdk16/pull/53/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk16&pr=53&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258007
  Stats: 10 lines in 2 files changed: 6 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk16/pull/53.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16 pull/53/head:pull/53

PR: https://git.openjdk.java.net/jdk16/pull/53