RFR: 8339170: ☂ Convert tests to JUnit 5

[Marius Hanl]

These are the remaining bits and pieces in order to completely remove the JUnit Vintage Engine, and therefore JUnit 4 from JavaFX.
After that, we should either document, that JUnit5 is used (just as information) or close [JDK-8296284](https://bugs.openjdk.org/browse/JDK-8296284) (Since you can not use JUnit4 anymore).

This also removes the `org.hamcrest` dependency, which is actually only used in one test where it can easily be replaced by a `Predicate` (instead of `Matcher`). I'm not convinced that we should keep that dependency for just one test.

Note: I could not get the `:swt` tests to compile/test, in order to run the tests. 
I need some guidance here how to instruct `Gradle` to compile this module (and if I need anything else? like swt?).

Also, this probably needs an extra ticket in the JBS?

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Commit messages:
 - JDK-8339170: ☂ Convert tests to JUnit 5

Changes: https://git.openjdk.org/jfx/pull/1774/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1774&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339170
  Stats: 240 lines in 9 files changed: 26 ins; 152 del; 62 mod
  Patch: https://git.openjdk.org/jfx/pull/1774.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1774/head:pull/1774

PR: https://git.openjdk.org/jfx/pull/1774