RFR: 8296556: Mark two RobotTest methods as unstable on HiDPI Windows systems

[Karthik P K]

Two RobotTest methods are marked as unstable on HiDPI Windows systems.

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Commit messages:
 - Rephrase comment of skipping test
 - Add BugID of the test failure bug
 - Mark tests as unstable for HiDPI on Windows

Changes: https://git.openjdk.org/jfx/pull/951/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=951&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296556
  Stats: 8 lines in 1 file changed: 8 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/951.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/951/head:pull/951

PR: https://git.openjdk.org/jfx/pull/951