<AWT Dev> RFR: 8270116: Expand ButtonGroupLayoutTraversalTest.java to run in all LaFs, including Aqua on macOS

[rajat mahajan]

Summary: Expanded ButtonGroupLayoutTraversalTest.java to run in all LAFs on all OS. Added synchronization for focusCnt.

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Commit messages:
 - 8270116: Expand ButtonGroupLayoutTraversalTest.java to run in all LaFs, including Aqua on macOS

Changes: https://git.openjdk.java.net/jdk/pull/4937/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4937&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8270116
  Stats: 112 lines in 1 file changed: 44 ins; 28 del; 40 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4937.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4937/head:pull/4937

PR: https://git.openjdk.java.net/jdk/pull/4937