RFR: 8300084: AquaFileChooserUI.getDefaultButton returns null

[Tejesh R]

In Aqua look and feel, `getDefaultButton()` of `JFileChooser` returns null. The fix overrides the method from `FileChooserUI` to return the `ApproveButton` of `FileChooser` which is similar to other L&F. 
The test is run for multiple iterations in CI machine.

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Commit messages:
 - fix + test

Changes: https://git.openjdk.org/jdk/pull/12008/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12008&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300084
  Stats: 58 lines in 2 files changed: 58 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12008.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12008/head:pull/12008

PR: https://git.openjdk.org/jdk/pull/12008