RFR: 8286788: Test java/lang/Thread/virtual/ThreadAPI.testGetStackTrace3 fails

[Alan Bateman]

This is a test fix. ThreadAPI.testGetStackTrace3 tests Thread::getStackTrace on a thread doing a selection operation. The test is not reliable as it expects to see the "select" method in the stack trace after waiting 200ms. The test is changed to poll the stack trace so that it's no longer dependent on sleep.

The update includes a drive-by change to the test description to use `@enablePreview`.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/8743/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8743&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286788
  Stats: 10 lines in 1 file changed: 3 ins; 2 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8743.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8743/head:pull/8743

PR: https://git.openjdk.java.net/jdk/pull/8743