RFR: 8328366: Thread.setContextClassloader from thread in FJP commonPool task no longer works after JDK-8327501

[Viktor Klang]

This is a draft PR with a potential solution to 8328366 without regressing 8327501.

@DougLea To avoid regressions in the future, where would regression tests for this ideally be located?

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Commit messages:
 - Adding test case to verify that it is possible to change the CCL of the common pool
 - Restore setContextClassLoader functionality without breaking JDK-8327501

Changes: https://git.openjdk.org/jdk/pull/18374/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18374&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328366
  Stats: 14 lines in 2 files changed: 8 ins; 5 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/18374.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18374/head:pull/18374

PR: https://git.openjdk.org/jdk/pull/18374