RFR: 8296284: Update CONTRIBUTING guidelines to state that JUnit 5 is used for tests

[Marius Hanl]

As written in the ticket, we might want to clarify that we use `JUnit5` for testing in the `CONTRIBUTING` guidelines.

This is a first idea of how we could document this and also link to the JUnit page in case contributors are not familiar with the framework (or `JUnit5` in particular). 

Feedback welcome!

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Commit messages:
 - 8296284: Update CONTRIBUTING guidelines to state that JUnit 5 is used for tests

Changes: https://git.openjdk.org/jfx/pull/1828/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1828&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296284
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1828.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1828/head:pull/1828

PR: https://git.openjdk.org/jfx/pull/1828