RFR: 8341982: Simplify JButton/bug4323121.java [v4]

[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:

  Remove space before comma
  
  Co-authored-by: Abhishek Kumar <abhishek.cx.kumar at oracle.com>

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/21475/files
  - new: https://git.openjdk.org/jdk/pull/21475/files/f32b1f57..8759e936

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21475&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21475&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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