<AWT Dev> RFR: 8233565: [TESTBUG] NullModalityDialogTest.java fails on MacOS

[Prasanta Sadhukhan]

This test NullModalityDialogTest.java was failing repeatedly on macos in automated testing citing "Tab navigation did not happen properly". 
Modified to use invokeAndWait() instead of invokeLater and also use setAutoDelay() with appropriate delay time, in consistent with other automated tests running on mach5.
Several iterations of the test works fine in all platforms after this modification.

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Commit messages:
 - 8233565: [TESTBUG] NullModalityDialogTest.java fails on MacOS

Changes: https://git.openjdk.java.net/jdk/pull/3488/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3488&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8233565
  Stats: 7 lines in 2 files changed: 3 ins; 1 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3488.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3488/head:pull/3488

PR: https://git.openjdk.java.net/jdk/pull/3488