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

[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`

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Commit messages:
 - 8269478: Shenandoah: gc/shenandoah/mxbeans tests should fail when no notifications arrive

Changes: https://git.openjdk.java.net/jdk/pull/4611/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4611&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269478
  Stats: 8 lines in 2 files changed: 4 ins; 0 del; 4 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