RFR: 8328665: serviceability/jvmti/vthread/PopFrameTest failed with a timeout [v7]

[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 incrementally with one additional commit since the last revision:

  review: adjusted trap timeout and fixed a typo in comment

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18419/files
  - new: https://git.openjdk.org/jdk/pull/18419/files/f8344149..21c57eae

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18419&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18419&range=05-06

  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 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