RFR: 8292679: Simplify thread creation in gtest and port 2 tests to new way

[Johan Sjölén]

This PR removes a lot of boilerplate that was required when creating multi-threaded tests and ports 2 tests to show that the number of lines of code is significantly reduced and that porting is fairly trivial. The ported tests also serve as an introduction to others who're looking into writing their own multi-threaded tests.

Testing: Ran the ported tests on my machine, also running the tests on x64 {Windows, Linux, Mac OS X],  aarch64 { Linux, Mac OS X} and tier1 for "completeness".

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Commit messages:
 - Implement new thread helpers

Changes: https://git.openjdk.org/jdk/pull/9962/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9962&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292679
  Stats: 199 lines in 3 files changed: 80 ins; 73 del; 46 mod
  Patch: https://git.openjdk.org/jdk/pull/9962.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9962/head:pull/9962

PR: https://git.openjdk.org/jdk/pull/9962