RFR: 8316154: Opensource JTextArea manual tests

[Prasanta Sadhukhan]

JTextArea manual applet test converted to automated and opensourced

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Commit messages:
 - 8316154: Opensource JTextArea manual tests

Changes: https://git.openjdk.org/jdk/pull/15700/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15700&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316154
  Stats: 90 lines in 1 file changed: 90 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15700.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15700/head:pull/15700

PR: https://git.openjdk.org/jdk/pull/15700