RFR: 8327829: [JVMCI] runtime/ClassUnload/ConstantPoolDependsTest.java fails on libgraal

[Doug Simon]

This PR fixes failures of `runtime/ClassUnload/ConstantPoolDependsTest.java` for similar reasons to those stated in https://github.com/openjdk/jdk/pull/18044.

The fix is also similar: use a helper that retries class unloading.

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Commit messages:
 - allow more time for class unloading in ConstantPoolDependsTest

Changes: https://git.openjdk.org/jdk/pull/18202/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18202&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327829
  Stats: 8 lines in 1 file changed: 2 ins; 3 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/18202.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18202/head:pull/18202

PR: https://git.openjdk.org/jdk/pull/18202