RFR: 8296284: Update CONTRIBUTING guidelines to state that JUnit 5 is used for tests [v2]

[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!

Marius Hanl has updated the pull request incrementally with one additional commit since the last revision:

  Move JUnit5 reference to 'Test your changes'

-------------

Changes:
  - all: https://git.openjdk.org/jfx/pull/1828/files
  - new: https://git.openjdk.org/jfx/pull/1828/files/fb31f897..a78926ef

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1828&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1828&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 1 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