[jdk11u-dev] RFR: 8280913: Create a regression test for JRootPane.setDefaultButton() method

[Goetz Lindenmaier]

I backport this for parity with 11.0.17-oracle.

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Commit messages:
 - Backport 51b53a821bb3cfb962f80a637f5fb8cde988975a

Changes: https://git.openjdk.org/jdk11u-dev/pull/1285/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=1285&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8280913
  Stats: 111 lines in 1 file changed: 111 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/1285.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev pull/1285/head:pull/1285

PR: https://git.openjdk.org/jdk11u-dev/pull/1285