RFR: 8341982: Simplify JButton/bug4323121.java

[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.

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Commit messages:
 - 8341982: Simplify JButton/bug4323121.java

Changes: https://git.openjdk.org/jdk/pull/21475/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21475&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341982
  Stats: 90 lines in 1 file changed: 37 ins; 32 del; 21 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