RFR: JDK-8299052 : ViewportOverlapping test fails intermittently on Win10 & Win11

[Harshitha Onkar]

ViewportOverlapping test was failing intermittently during CI runs on Win10 and Win11. 

Although this intermittent issue wasn't replicable when the individual test as well as the test group was run multiple times, a defensive fix has been applied. The fix is made to the base class -OverlappingTestBase.java, which holds the common Standard Test Machinery for AWT_Mixing tests.

 `Toolkit.getDefaultToolkit().sync()` is added `OverlappingTestBase.testComponent()`. I believe Thread.sleep(500) alone, might not be sufficient to flush all the graphics events of this test on slower systems which might be causing this test to fail intermittently during CI runs.

Screenshot capture has been added to ViewportOverlapping.java to aid debugging this intermittent issue, in case it should occur again. 

PS: Only relevant code in OverlappingTestBase has been cleaned-up as required by this test. There are other parts of the base class that require code clean-up  which is best if taken up as a separate issue as it has common code across many AWT_Mixing tests.

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Commit messages:
 - exapnded imports
 - initial test fix

Changes: https://git.openjdk.org/jdk/pull/14289/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14289&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299052
  Stats: 112 lines in 3 files changed: 54 ins; 13 del; 45 mod
  Patch: https://git.openjdk.org/jdk/pull/14289.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14289/head:pull/14289

PR: https://git.openjdk.org/jdk/pull/14289