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

[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

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Commit messages:
 - 8328665: serviceability/jvmti/vthread/PopFrameTest failed with a timeout

Changes: https://git.openjdk.org/jdk/pull/18419/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18419&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328665
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 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