RFR: 8361298: SwingUtilities/bug4967768.java fails where character P is not underline

[Abhishek Kumar]

For Windows and GTK L&F, mnemonic is visible when ALT key is pressed.
Test instruction updated to reflect the desired behaviour .

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Commit messages:
 - Test instruction update

Changes: https://git.openjdk.org/jdk/pull/26546/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26546&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8361298
  Stats: 9 lines in 1 file changed: 6 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/26546.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26546/head:pull/26546

PR: https://git.openjdk.org/jdk/pull/26546