RFR: 8321625: ContextMenuNPETest fails intermittently on Linux

[Ziad El Midaoui]

The test seems to be working in Linux currently, so I enabled it for Linux systems.

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Commit messages:
 - Removed unused imports
 - Update ContextMenuNPETest.java
 - Added testWatcher and enabled test for Linux systems
 - Merge branch 'openjdk:master' into 8321625.ContextMenuNPETest
 - test issue

Changes: https://git.openjdk.org/jfx/pull/1893/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1893&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321625
  Stats: 5 lines in 1 file changed: 0 ins; 5 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1893.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1893/head:pull/1893

PR: https://git.openjdk.org/jfx/pull/1893