[jdk21u-dev] RFR: 8328562: Convert java/awt/InputMethods/DiacriticsTest/DiacriticsTest.java applet test to main

[Goetz Lindenmaier]

I backport this for parity with 21.0.10-oracle.

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Commit messages:
 - Backport fb8f2a0a929ebe7f65c69741712b89bbb403ade9

Changes: https://git.openjdk.org/jdk21u-dev/pull/2184/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=2184&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328562
  Stats: 137 lines in 2 files changed: 64 ins; 57 del; 16 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/2184.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/2184/head:pull/2184

PR: https://git.openjdk.org/jdk21u-dev/pull/2184