RFR: 8294697: java/lang/Thread/virtual/ThreadAPI.testGetStackTrace2 failed with non-empty stack trace

[Alan Bateman]

This is a test-only change. ThreadAPI.testGetStackTrace2 tests the stack trace of a virtual thread that has started but has not executed it. The test currently attempts to saturate all carrier threads before starting a target thread. The test is not reliable and failed at least once with a debug build and -Xcomp. The test is changed to use a custom scheduler that discards the task so the thread doesn't execute. The resulting test is much simpler and removes 3s from the execution time.

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

Commit messages:
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/10576/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10576&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294697
  Stats: 36 lines in 1 file changed: 0 ins; 30 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/10576.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10576/head:pull/10576

PR: https://git.openjdk.org/jdk/pull/10576