RFR: 8367898: Skip StageFocusTest on Linux

[Kevin Rushforth]

Skip `StageFocusTest` on Linux until [JDK-8367893](https://bugs.openjdk.org/browse/JDK-8367893) is fixed.

Also tested on macOS and Windows to verify that the test is still run.

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Commit messages:
 - 8367898: Skip StageFocusTest on Linux

Changes: https://git.openjdk.org/jfx/pull/1909/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1909&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367898
  Stats: 6 lines in 1 file changed: 6 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1909.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1909/head:pull/1909

PR: https://git.openjdk.org/jfx/pull/1909