<AWT Dev> RFR: 8233567: [TESTBUG] FocusSubRequestTest.java fails on macos

[Prasanta Sadhukhan]

This test was problemlisted during nightly testing of jdk on macos which seems to be caused by the fact that proper waiting was not done to make sure frame is visible before commencing the test, which can be problematic in some of the slower CI macos system.
Modified the wait-till-frame-is-visible code to make it more in sync with what other similar test use.
Several iterations of the test works fine in all platforms after this modification.

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Commit messages:
 - 8233567: [TESTBUG] FocusSubRequestTest.java fails on macos

Changes: https://git.openjdk.java.net/jdk/pull/3430/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3430&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8233567
  Stats: 13 lines in 2 files changed: 3 ins; 2 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3430.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3430/head:pull/3430

PR: https://git.openjdk.java.net/jdk/pull/3430