RFR: 8321626: [testbug] Mark DualWindowTest and ContextMenuNPETest unstable on Linux

[Kevin Rushforth]

Until JDK-8321624 and JDK-8321625 are fixed, mark the following two tests as unstable on Linux, meaning they will not be run as part of our nightly headful test runs.

DualWindowTest - [JDK-8321624](https://bugs.openjdk.org/browse/JDK-8321624)
ContextMenuNPETest - [JDK-8321625](https://bugs.openjdk.org/browse/JDK-8321625)

These two tests are causing a lot of noise in our CI headful test runs. By skipping these two tests, we get clean test runs.

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Commit messages:
 - 8321626: [testbug] Mark DualWindowTest and ContextMenuNPETest unstable on Linux

Changes: https://git.openjdk.org/jfx/pull/1302/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1302&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321626
  Stats: 14 lines in 2 files changed: 14 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1302.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1302/head:pull/1302

PR: https://git.openjdk.org/jfx/pull/1302