[jdk11u-dev] RFR: 8269478: Shenandoah: gc/shenandoah/mxbeans tests should be more resilient

[Aleksey Shipilev]

Clean backport to improve test stability.

Additional testing:
 - [x] Affected tests still pass

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

Commit messages:
 - Backport 23d2996fee54afcc386a505a65d3236aab874e9e

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/258/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=258&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269478
  Stats: 97 lines in 2 files changed: 68 ins; 18 del; 11 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/258.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/258/head:pull/258

PR: https://git.openjdk.java.net/jdk11u-dev/pull/258