RFR: 8347836: Disabled PopupMenu shows shortcuts on Mac [v3]

[Damon Nguyen]

> The test instructions say that disabled PopupMenus should not have shortcuts shown, but on MacOS, these shortcuts still appear. When checking native MacOS15 behavior, disabled PopupMenus still have shortcuts shown. Since the test doesn't modify the popup's shortcuts other than adding the shortcut for `A`, it makes sense that the result matches native behavior. So, I modified the test instructions instead to exclude MacOS from this step.

Damon Nguyen has updated the pull request incrementally with two additional commits since the last revision:

 - Separate comment blocks
 - Add platform conditionals to test instructions

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Changes:
  - all: https://git.openjdk.org/jdk/pull/23402/files
  - new: https://git.openjdk.org/jdk/pull/23402/files/1cc6c40e..fb78b9ef

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23402&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23402&range=01-02

  Stats: 14 lines in 1 file changed: 7 ins; 2 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/23402.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23402/head:pull/23402

PR: https://git.openjdk.org/jdk/pull/23402