RFR: 8338282: javax/swing/JMenuBar/TestMenuMnemonicLinuxAndMac.java test failed on macOS and Ubuntu

[Abhishek Kumar]

Test is failing on intermittently on macOS and ubuntu machine in CI testing.
Test stabilization done by requesting focus for the frame before starting the testing. 

Ran the test before and after the fix and there is no failure after test stabilization for multiple run.
CI link is added in JBS.

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Commit messages:
 - Test stabilization

Changes: https://git.openjdk.org/jdk/pull/26307/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26307&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338282
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/26307.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26307/head:pull/26307

PR: https://git.openjdk.org/jdk/pull/26307