RFR: 8065097: [macosx] javax/swing/Popup/TaskbarPositionTest.java fails because Popup is one pixel off

[Prasanta Sadhukhan]

Test seems to fail in macOS citing popup location is 1 pixel off compared to combobox position ie
`p.y 154 cpos.y 155`

But in macOS, popup location, if 1st entry is selected, is 1 pixel higher than combobox so it's expected as can be seen below
<img width="661" alt="Screenshot 2022-06-29 at 1 53 42 PM" src="https://user-images.githubusercontent.com/43534309/176389227-4e568117-ffce-4b4a-92d7-ab5c78c4ce08.png">

so test is updated to add 1 pixel to popup location. 
It will not affect windows and linux as popup is placed below the combobox in those platforms so 1 pixel higher will not make any difference.
Also, added some stability fixes..Test passed for several iterations in all platforms in CI..

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Commit messages:
 - 8065097: [macosx] javax/swing/Popup/TaskbarPositionTest.java fails because Popup is one pixel off
 - 8065097: [macosx] javax/swing/Popup/TaskbarPositionTest.java fails because Popup is one pixel off
 - 8065097: [macosx] javax/swing/Popup/TaskbarPositionTest.java fails because Popup is one pixel off

Changes: https://git.openjdk.org/jdk/pull/9321/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9321&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8065097
  Stats: 128 lines in 2 files changed: 36 ins; 1 del; 91 mod
  Patch: https://git.openjdk.org/jdk/pull/9321.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9321/head:pull/9321

PR: https://git.openjdk.org/jdk/pull/9321