RFR: 8300084: AquaFileChooserUI.getDefaultButton returns null [v2]

[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.

Tejesh R has updated the pull request incrementally with one additional commit since the last revision:

  Updated based on review comments

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12008/files
  - new: https://git.openjdk.org/jdk/pull/12008/files/59a79632..81d2cda2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12008&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12008&range=00-01

  Stats: 6 lines in 2 files changed: 1 ins; 0 del; 5 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