RFR: 8307080: Open source some more JComboBox jtreg tests

[Phil Race]

Open source another 5 JComboBox tests.

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Commit messages:
 - 8307080
 - 8307080

Changes: https://git.openjdk.org/jdk/pull/13731/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13731&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/jdk/pull/13731.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13731/head:pull/13731

PR: https://git.openjdk.org/jdk/pull/13731