RFR: 8262271: SA: Add new stress test that tests getting the stack trace of an active thread

[Chris Plummer]

This a new test that runs jstack on a process that is busy doing things. It runs jstack 4 times as the process is starting up.

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Commit messages:
 - Fix @bug
 - Add JStackStressTest

Changes: https://git.openjdk.java.net/jdk/pull/2720/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2720&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8262271
  Stats: 115 lines in 1 file changed: 115 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2720.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2720/head:pull/2720

PR: https://git.openjdk.java.net/jdk/pull/2720