RFR: 8356690: Update JUnit to 5.12.2

[Marius Hanl]

This PR bumps the JUnit version to 5.12.2.
Also removed 2 now unused variables since I phased out the JUnit Vintage Engine + Hamcrest.

Changelog: https://junit.org/junit5/docs/current/release-notes/index.html#release-notes-5.12.2
The mentioned deprecations do not affect us.

Interesting new features:
- Thread dumps on test timeouts
- Parameterized test validation improvements
- JRE-based conditions such as @EnabledOnJre and @DisabledForJreRange now support arbitrary Java versions

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Commit messages:
 - 8356690: Update JUnit to 5.12.2

Changes: https://git.openjdk.org/jfx/pull/1803/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1803&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8356690
  Stats: 44 lines in 2 files changed: 0 ins; 2 del; 42 mod
  Patch: https://git.openjdk.org/jfx/pull/1803.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1803/head:pull/1803

PR: https://git.openjdk.org/jfx/pull/1803