RFR: 8341982: Simplify JButton/bug4323121.java [v8]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 10 additional commits since the last revision:

 - Merge master
 - Update summary and error message
 - Get rid of windowGainedFocus latch
 - Use JButton directly instead of TestButton subclass
 - Remove space before comma
   
   Co-authored-by: Abhishek Kumar <abhishek.cx.kumar at oracle.com>
 - Amend indentation in the finally block
 - Inline mouseEntered listener for the button
 - Clarify thread-safety of the modelArmed flag
 - Remove MouseMotionListener; get rid of eventHandler variable
 - 8341982: Simplify JButton/bug4323121.java

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/21475/files
  - new: https://git.openjdk.org/jdk/pull/21475/files/d61e4331..3558f0c5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21475&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21475&range=06-07

  Stats: 246583 lines in 2680 files changed: 148578 ins; 72734 del; 25271 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