RFR: 8328402: Implement pausing functionality for the PassFailJFrame

[Alexander Zvegintsev]

> we need to add next to Pass/Fail a "Pause Timer" button, that
(a) stops the count down
(b) changes the Pause Timer to "Resume Timer"
~~(c) disables Pass/Fail until the timer is resumed~~
the test will not have to pause or be aware - only the PassFailJFrame machinery.
~~So the tester can do anything they want except exit the test in the paused 
mode.~~

This PR implements everything except the (c).
I see nothing wrong with completing the test from the paused state, and it does 
not add an extra click.  


Example screenshots:

![image](https://github.com/openjdk/jdk/assets/77687766/65d34bf8-1e46-4e68-b39e-a77f257ec3c0)
![image](https://github.com/openjdk/jdk/assets/77687766/3a43e059-2d48-46f3-9d1b-c2ab84fcce37)

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Commit messages:
 - 8328402: Implement pausing functionality for the PassFailJFrame

Changes: https://git.openjdk.org/jdk/pull/18368/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18368&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328402
  Stats: 31 lines in 1 file changed: 27 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/18368.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18368/head:pull/18368

PR: https://git.openjdk.org/jdk/pull/18368