RFR: 8267621: Mark HonorDeveloperSettingsTest.testOpacityWithManuallyChangedValueAndInlineStyleIsSetToInlineStyle as unstable

[Ambarish Rapte]

The test HonorDeveloperSettingsTest.testOpacityWithManuallyChangedValueAndInlineStyleIsSetToInlineStyle is failing intermittently only on GHA build on Linux platform. see: [JDK-8267425](https://bugs.openjdk.java.net/browse/JDK-8267425)

A fix proposed at #509 is a potential fix which adds cleanup of test. But we do not know the root cause of this failure.
Identifying the root cause may take longer which we might push that to next test sprint and the test failure results in failing the Github checks on PR.  So we need to mark this test as unstable.
As of now the test fails only on Linux so we will mark it unstable only for Linux.

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Commit messages:
 - mark unstable

Changes: https://git.openjdk.java.net/jfx/pull/517/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=517&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267621
  Stats: 6 lines in 1 file changed: 6 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jfx/pull/517.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/517/head:pull/517

PR: https://git.openjdk.java.net/jfx/pull/517