RFR: 8309241: ClassForNameLeak fails intermittently as the class loader hasn't been unloaded

[Mandy Chung]

Convert `ClassForNameLeak` test to use `jdk.test.lib.util.ForceGC` that would be more reliable in waiting for the class loader to be unloaded.

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Commit messages:
 - 8309241: ClassForNameLeak fails intermittently as the class loader hasn't been unloaded

Changes: https://git.openjdk.org/jdk/pull/14269/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14269&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309241
  Stats: 58 lines in 1 file changed: 15 ins; 29 del; 14 mod
  Patch: https://git.openjdk.org/jdk/pull/14269.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14269/head:pull/14269

PR: https://git.openjdk.org/jdk/pull/14269