RFR: 8269478: Shenandoah: gc/shenandoah/mxbeans tests should be more resilient [v4]

[Aleksey Shipilev]

On Mon, 28 Jun 2021 17:08:39 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> After JDK-8259954, some mxbeans tests wait in the effectively infinite loop for the notifications to arrive. If notifications are broken, it would wait forever, and test would timeout. We need to make such failure mode more straightforward.
>> 
>> Additional testing: 
>>  - [x] Linux x86_64 fastdebug `gc/shenandoah/mxbeans` 
>>  - [x] Linux x86_64 fastdebug `gc/shenandoah/mxbeans`, `-Xcomp`
>
> Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:
> 
>   More comments

Rewrote the `PauseNotification` test to capture notification counts, not pause times. Mostly because the 1ms resolution is not enough to guarantee non-zero sum of pauses.

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

PR: https://git.openjdk.java.net/jdk/pull/4611