RFR: 8215948: [TESTBUG] gtest pseudo-JavaThreads could be more regular JavaThreads

[David Holmes]

Please review this change to the gtest infrastructure that makes the test JavaThreads more regular JavaThreads, with the same lifecycle methods and having a regular j.l.Thread object.

One test had to be modified slightly to create and start the threads outside of a code region where a Mutex is held.

Testing: tiers 1-3, GHA

Thanks,
David

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Commit messages:
 - 8215948: [TESTBUG] gtest pseudo-JavaThreads could be more regular JavaThreads

Changes: https://git.openjdk.java.net/jdk/pull/4704/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4704&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8215948
  Stats: 108 lines in 2 files changed: 43 ins; 38 del; 27 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4704.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4704/head:pull/4704

PR: https://git.openjdk.java.net/jdk/pull/4704