RFR: 8341982: Simplify JButton/bug4323121.java [v7]

[Alexey Ivanov]

> The test `javax/swing/JButton/bug4323121.java` contains lots of unused methods.
> 
> I removed all the unused methods by extending `MouseAdapter`.
> 
> I use `CountDownLatch` to synchronise actions in the test.
> 
> The test still verifies `button.getModel().isArmed()` doesn't always return `true` for classes which extend `JButton`. I verified the updated test fails in 1.3.0 and passes in 1.4.0, so the test still reproduces the original problem.

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

  Update summary and error message

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Changes:
  - all: https://git.openjdk.org/jdk/pull/21475/files
  - new: https://git.openjdk.org/jdk/pull/21475/files/d16918a4..d61e4331

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21475&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21475&range=05-06

  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/21475.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21475/head:pull/21475

PR: https://git.openjdk.org/jdk/pull/21475