RFR: 8259954: gc/shenandoah/mxbeans tests fail with -Xcomp

[Aleksey Shipilev]

See the bug report for initial observation. The key thing is that the asynchronous GC notifications can arrive late, and they do arrive late with `-Xcomp`, because all that code is now waiting for compilation. The answer is to wait a bit smarter.

Additional testing:
 - [x] `gc/shenandoah/mxbeans` default mode
 - [x] `gc/shenandoah/mxbeans` with `-Xcomp`
 - [x] `gc/shenandoah/mxbeans` with `-Xint`

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Commit messages:
 - 8259954: gc/shenandoah/mxbeans tests fail with -Xcomp

Changes: https://git.openjdk.java.net/jdk/pull/2179/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2179&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259954
  Stats: 12 lines in 2 files changed: 10 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2179.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2179/head:pull/2179

PR: https://git.openjdk.java.net/jdk/pull/2179