RFR: 8328697: SubMenuShowTest and SwallowKeyEvents tests stabilization

[Alexander Zvegintsev]

Two tests may fail intermittently on Wayland, the standard `waitForIdle` and `delay` fixes the issue.

Manual and CI testing looks good.

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Commit messages:
 - initial

Changes: https://git.openjdk.org/jdk/pull/18431/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18431&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328697
  Stats: 6 lines in 2 files changed: 4 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/18431.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18431/head:pull/18431

PR: https://git.openjdk.org/jdk/pull/18431