RFR: 8310072: JComboBox/DisabledComboBoxFontTestAuto: Enabled and disabled ComboBox does not match in these LAFs: GTK+

[Abhishek Kumar]

Test was failing on GTK and Windows LAF due to pixel color mismatch. Th reason behind this issue was the size of image which is different and that results in incorrect pixel comparison. Fix is to ensure that correct pixel is matched and the pixel color should remain within tolerance. 
For windows LAF the background color is not an exact match and thus added a TOLERANCE field to check if the RGB difference is within limits.

`@key headful` added in jtreg tag to ensure that test run for GTK LAF as well which was not the case before as it is mentioned in JBS `It does not fail in mach5 because on linux + headless mode the gtk L&F is not supported.`

CI testing is green for the modified test. Link attached in JBS.

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Commit messages:
 - ComboBox color issue fix

Changes: https://git.openjdk.org/jdk/pull/18644/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18644&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310072
  Stats: 36 lines in 1 file changed: 22 ins; 6 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/18644.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18644/head:pull/18644

PR: https://git.openjdk.org/jdk/pull/18644