RFR: 8326121: vmTestbase/gc/g1/unloading/tests/unloading_keepRef_rootClass_inMemoryCompilation_keep_cl failed with Full gc happened. Test was useless.

[Ivan Walulya]

Hi all,

Please review this change to trigger SkippedException instead of RunTimeExceptions when a test is deemed useless because of full-GCs. The test triggers frequent `System.gc()` calls which in some cases may aggressively shrink the heap, and subsequent allocations lead to full-GC. This may affect the usefulness of the test, but it is not a test failure. So we added  SkippedException to indicate this situation.

Testing: Local testing with very small heaps.
              Tier 1.

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Commit messages:
 - import order
 - Skip test instead of flagging error

Changes: https://git.openjdk.org/jdk/pull/19432/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19432&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8326121
  Stats: 5 lines in 2 files changed: 4 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/19432.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19432/head:pull/19432

PR: https://git.openjdk.org/jdk/pull/19432