RFR: 8269478: Shenandoah: gc/shenandoah/mxbeans tests should fail when no notifications arrive [v3]

[Aleksey Shipilev]

> 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:

  Rework tests for much better reliability, passes slowdebug+Xcomp

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4611/files
  - new: https://git.openjdk.java.net/jdk/pull/4611/files/a00be3e6..029ed18f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4611&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4611&range=01-02

  Stats: 92 lines in 2 files changed: 45 ins; 28 del; 19 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4611.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4611/head:pull/4611

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