[jdk11u-dev] RFR: 8306850: Open source AWT Modal related tests

[Antonio Vieiro]

Clean backport of [JDK-8306850](https://bugs.openjdk.org/browse/JDK-8306850) for parity with Oracle 11.0.24.

Tested on Linux/Gnome and Windows 11. All tests pass.

-------------

Commit messages:
 - Backport 41ba05e450ed4467535bcee81946099e38665c3f

Changes: https://git.openjdk.org/jdk11u-dev/pull/2791/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2791&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306850
  Stats: 1053 lines in 6 files changed: 1053 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/2791.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2791/head:pull/2791

PR: https://git.openjdk.org/jdk11u-dev/pull/2791