RFR: 8267425: Intermittent failure of HonorDeveloperSettingsTest unit test

[Ambarish Rapte]

The same failure was earlier fixed at PR #496, which seems insufficient.
The changes in this PR were also tried ealier in the PR #496 but later removed, as they seemed not necessary.
Fix is to clean up the test, by removing all children added to root and hiding the stage.
The test only fails on the GHA run, if it continues to fail after this fix then we shall skip it till next test sprint.

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Commit messages:
 - root.getChildren().clear();  stage.hide();

Changes: https://git.openjdk.java.net/jfx/pull/509/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=509&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267425
  Stats: 14 lines in 1 file changed: 10 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jfx/pull/509.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/509/head:pull/509

PR: https://git.openjdk.java.net/jfx/pull/509