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.
------------- 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
