RFR: 8289184: runtime/ClassUnload/DictionaryDependsTest.java failed with "Test failed: should be unloaded"

[Coleen Phillimore]

Some allocation and System.gc() isn't sufficient to cause class unloading.  It works almost all the time, but it's better to call WhiteBox.fullGC() from ClassUnloadCommon.triggerUnloading().  This requires building and copying WhiteBox classes to the test.classes directory.
Tested with tier1-4.

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Commit messages:
 - 8289184: runtime/ClassUnload/DictionaryDependsTest.java failed with "Test failed: should be unloaded"

Changes: https://git.openjdk.org/jdk/pull/9435/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9435&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289184
  Stats: 24 lines in 5 files changed: 13 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/9435.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9435/head:pull/9435

PR: https://git.openjdk.org/jdk/pull/9435