RFR: 8319161: GC: Make TestParallelGCThreads use createTestJavaProcessBuilder

[Ramkumar Sunderbabu]

Summary of change:

1. executeLimitedTestJava changed to executeTestJava to allow flags to be propagated to forked processes.
2. `@requires vm.gc == null` - this line is added to skip runs that use GC flags in test vm options. Allowing GC flags will result in VM initialization error ("Multiple garbage collectors selected").
3. Addition of ZGC to list of parallel collectors

Testing:
HS Tiers 1 and 2
Ran with all flag combinations in CI

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Commit messages:
 - initial commit

Changes: https://git.openjdk.org/jdk/pull/28566/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28566&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319161
  Stats: 10 lines in 1 file changed: 4 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/28566.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28566/head:pull/28566

PR: https://git.openjdk.org/jdk/pull/28566