RFR: 8286846: test/jdk/javax/swing/plaf/aqua/CustomComboBoxFocusTest.java fails on mac aarch64

[Prasanta Sadhukhan]

Test was failing intermiitently in iMac M1 system owing to minimalistic color difference of 1 

x 13 y 0 refRGB ffeeeeee customRGB ffefeeee
x 0 y 0 refRGB ffefefef customRGB ffefeeef


so added color tolerance check. 
Also, added other stability fixes..
Several iteration of test passed in intended M1 and x64 system.

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Commit messages:
 - 8286846: test/jdk/javax/swing/plaf/aqua/CustomComboBoxFocusTest.java fails on mac aarch64
 - 8286846: test/jdk/javax/swing/plaf/aqua/CustomComboBoxFocusTest.java fails on mac aarch64

Changes: https://git.openjdk.java.net/jdk/pull/8864/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8864&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286846
  Stats: 38 lines in 1 file changed: 20 ins; 8 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8864.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8864/head:pull/8864

PR: https://git.openjdk.java.net/jdk/pull/8864