RFR: 8328562: Convert java/awt/InputMethods/DiacriticsTest/DiacriticsTest.java applet test to main

[Alexander Zvegintsev]

Test converted to main.
Instructions split to Windows and Linux, with some clarifications.
The test runs successfully on Windows and Linux.

-------------

Commit messages:
 - split instructions for Windows and Linux
 - initial

Changes: https://git.openjdk.org/jdk/pull/18388/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18388&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328562
  Stats: 137 lines in 2 files changed: 65 ins; 57 del; 15 mod
  Patch: https://git.openjdk.org/jdk/pull/18388.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18388/head:pull/18388

PR: https://git.openjdk.org/jdk/pull/18388