[jdk21u-dev] RFR: 8327751: Convert javax/swing/JInternalFrame/6726866/bug6726866.java applet test to main

[Goetz Lindenmaier]

I backport this for parity with 21.0.9-oracle

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Commit messages:
 - Backport cfd9209e03176bd8e02acd74b51a16f3113fbd21

Changes: https://git.openjdk.org/jdk21u-dev/pull/1887/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1887&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327751
  Stats: 59 lines in 2 files changed: 19 ins; 32 del; 8 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1887.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1887/head:pull/1887

PR: https://git.openjdk.org/jdk21u-dev/pull/1887