RFR: 8339979: VirtualThreadSchedulerMXBeanTest.testReduceParallelism fails intermittently [v2]

[Alan Bateman]

> This is a test only change. VirtualThreadSchedulerMXBeanTest.testReduceParallelism can fail when getMountedVirtualThreadCount over estimates the number of mounted virtual threads.
> 
> The test is changed from using getMountedVirtualThreadCount to sample the result until it reads the expected value.
> 
> As part of the change, the awaitXXX supporting methods are refactored to take a predicate and the trace message.

Alan Bateman has updated the pull request incrementally with one additional commit since the last revision:

  Bring back Gte/Lte methods, reduces changes to one test

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/21310/files
  - new: https://git.openjdk.org/jdk/pull/21310/files/d65a371c..be9e8b92

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21310&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21310&range=00-01

  Stats: 56 lines in 1 file changed: 43 ins; 2 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/21310.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21310/head:pull/21310

PR: https://git.openjdk.org/jdk/pull/21310