[jdk11u-dev] RFR: 8294148: Support JSplitPane for instructions and test UI

[Andrew Lu]

I backport this for parity with 11.0.25-oracle.

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Commit messages:
 - Backport b419e9517361ed9d28f8ab2f5beacf5adfe3db91

Changes: https://git.openjdk.org/jdk11u-dev/pull/2741/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2741&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294148
  Stats: 239 lines in 1 file changed: 214 ins; 4 del; 21 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/2741.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2741/head:pull/2741

PR: https://git.openjdk.org/jdk11u-dev/pull/2741