[jdk11u-dev] RFR: 8307080: Open source some more JComboBox jtreg tests

[Andrew Lu]

I backport this for parity with 11.0.21-oracle.

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Commit messages:
 - Backport b8de39431dca90e63552968829a349a9b63e68ca

Changes: https://git.openjdk.org/jdk11u-dev/pull/2080/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2080&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307080
  Stats: 425 lines in 5 files changed: 425 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/2080.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2080/head:pull/2080

PR: https://git.openjdk.org/jdk11u-dev/pull/2080