RFR: 8266643: Intermittent failure of HonorDeveloperSettingsTest unit test

[Ambarish Rapte]

The test fails intermittently on github build. But I do not observe this issue locally on my Ubuntu20.04 VM.
I am suspecting that the reason of failure is that the test are not cleaned up correctly. The changes in this PR cleans up the test, by removing all children added to root and hiding the stage.

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Commit messages:
 - keep only SM.forget(scene), revert other changes
 - try SM.forget(scene)
 - reset css style manager before clearing scene
 - reset to original
 - skip setScene(null)
 - change order
 - css_test_cleanup_fix

Changes: https://git.openjdk.java.net/jfx/pull/496/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=496&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266643
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/496.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/496/head:pull/496

PR: https://git.openjdk.java.net/jfx/pull/496