RFR: 8328665: serviceability/jvmti/vthread/PopFrameTest failed with a timeout [v3]

[Serguei Spitsyn]

> This PR fixes a synchronization issue in the test:
>   `test/hotspot/jtreg/serviceability/jvmti/vthread/PopFrameTest`
>   
> The method `notifyAtBreakpoint()` can notify the `TestTask` thread when it has not reached an expected breakpoint yet.
> The fix is to add a call to the method `ensureAtBreakpoint()` one more time in the `B2` sub-test. It is needed after the top-most frame was popped with the JVMTI `PopFrame`, and the target thread needs to reach the breakpoint again after its execution was resumed.
> 
> The time is very intermittent. At least, I was not able to reproduce the timeout failure in thousands of mach5 runs with the `-Xcomp` option.
> 
> Testing:
>  - Run the test `test/hotspot/jtreg/serviceability/jvmti/vthread/PopFrameTest` thousands times in mach5

Serguei Spitsyn has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - “Merge”
 - review: updated test with one more call to notifyAtBreakpoint to reset the native state
 - 8328665: serviceability/jvmti/vthread/PopFrameTest failed with a timeout

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18419/files
  - new: https://git.openjdk.org/jdk/pull/18419/files/7843111d..1502492f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18419&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18419&range=01-02

  Stats: 61204 lines in 2409 files changed: 11744 ins; 7474 del; 41986 mod
  Patch: https://git.openjdk.org/jdk/pull/18419.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18419/head:pull/18419

PR: https://git.openjdk.org/jdk/pull/18419