RFR: 8339513: [TestBug] Convert fxml tests to JUnit 5

[Ajit Ghaisas]

This converts FXML module tests to junit5.

All changes are trivial API replacements from junit4 to junit5.
There are no parameterized tests in this module.

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Commit messages:
 - 8339513: Convert fxml tests to JUnit 5

Changes: https://git.openjdk.org/jfx/pull/1564/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1564&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339513
  Stats: 169 lines in 31 files changed: 45 ins; 16 del; 108 mod
  Patch: https://git.openjdk.org/jfx/pull/1564.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1564/head:pull/1564

PR: https://git.openjdk.org/jfx/pull/1564