RFR: 8315936: Parallelize gc/stress/TestStressG1Humongous.java test

[Ian Myers]

TestStressG1Humongous runs in hotspot:tier4 and takes about 900 seconds to run. It is one of the slowest tests in tier4, and it runs when concurrency is generally low due to exclusive adjacent tests. Thus, it limits the effective parallelism of tier4.

In test main method, there are a few independent test cases, which can be expressed in jtreg to gain parallel execution.

TestStressG1Humongous current runtime: **4917.51s user 410.97s system 549% cpu 16:10.07 total**
TestStressG1Humongous parallelized runtime: **4672.49s user 510.17s system 849% cpu 10:10.05 total**

This change takes the previous `run()` methods out of the main method and instead passes the parameters as part of the `@run` jtreg test annotation, allowing them to be run in parallel.

The last test runs the longest on its own, and this is why the runtime can't be reduced under the 10 minute mark.

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Commit messages:
 - Removing run() method and inlining with main()
 - Parallelize gc/stress/TestStressG1Humongous.java test

Changes: https://git.openjdk.org/jdk/pull/15943/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15943&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315936
  Stats: 46 lines in 1 file changed: 35 ins; 6 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/15943.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15943/head:pull/15943

PR: https://git.openjdk.org/jdk/pull/15943