RFR: 8287596: Reorg jdk.test.lib.util.ForceGC

[Xue-Lei Andrew Fan]

This is a follow up update per comments in [JDK-8287384 PR](https://github.com/openjdk/jdk/pull/8907).  The tier1 and tier2 test in open part looks good to me.  Please help to run Mach5 just case the closed test cases are impacted.

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Commit messages:
 - Remove trailing white space
 - 8287596: Reorg jdk.test.lib.util.ForceGC

Changes: https://git.openjdk.java.net/jdk/pull/8979/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8979&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287596
  Stats: 83 lines in 10 files changed: 12 ins; 40 del; 31 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8979.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8979/head:pull/8979

PR: https://git.openjdk.java.net/jdk/pull/8979