RFR: 8342524: Use latch in AbstractButton/bug6298940.java instead of delay [v2]

[Alexey Ivanov]

> Use a `CountDownLatch` in `javax/swing/AbstractButton/bug6298940.java` instead of delay.  
> Use `Util.hitMnemonics` instead of custom code to handle macOS.
> 
> I ran the updated test a few times with `JTREG=REPEAT_COUNT=20`, the test always passed in the CI.

Alexey Ivanov has updated the pull request incrementally with one additional commit since the last revision:

  Check for null before dispose()

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/23301/files
  - new: https://git.openjdk.org/jdk/pull/23301/files/c5c6b6a4..5969e82c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23301&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23301&range=00-01

  Stats: 5 lines in 1 file changed: 4 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/23301.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23301/head:pull/23301

PR: https://git.openjdk.org/jdk/pull/23301