RFR: 8353620: Make some systems tests robust for Ubuntu 24.04

[Jayathirth D V]

Under https://bugs.openjdk.org/browse/JDK-8339679 task, we are running full tests and identifying issues for Ubuntu 24.04.

As part of above task it is identified that there are 3 tests which fail intermittently and we need to add appropriate delays to make them more robust. Delays are added between UI launch and processing of UI events, with this change intermittent failures are not seen.

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Commit messages:
 - 8353620: Make some systems tests robust for Ubuntu 24.04

Changes: https://git.openjdk.org/jfx/pull/1761/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1761&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8353620
  Stats: 5 lines in 3 files changed: 3 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jfx/pull/1761.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1761/head:pull/1761

PR: https://git.openjdk.org/jfx/pull/1761