RFR: 8293282: LoadLibraryUnloadTest.java fails with "Too few cleared WeakReferences" [v2]

[Roger Riggs]

> Modify the LoadLibraryUnload test to call gc() more a few times, allowing multiple gc cycles to queue the expected refs.
> Short the timeout on each cycle to 10 sec.

Roger Riggs has updated the pull request incrementally with one additional commit since the last revision:

  Refactored to use test library ForceGC.wait waiting for all WeakReferences to be cleared.
  Cleanup/remove unnecessary LoadLibraryUnloadTest imports and -Xcheck:jni

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Changes:
  - all: https://git.openjdk.org/jdk/pull/10223/files
  - new: https://git.openjdk.org/jdk/pull/10223/files/fe704b58..498273d9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10223&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10223&range=00-01

  Stats: 17 lines in 2 files changed: 4 ins; 1 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/10223.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10223/head:pull/10223

PR: https://git.openjdk.org/jdk/pull/10223