RFR: 8319161: GC: Make TestParallelGCThreads use createTestJavaProcessBuilder [v2]

[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

Ramkumar Sunderbabu has updated the pull request incrementally with one additional commit since the last revision:

  separating test sections

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/28566/files
  - new: https://git.openjdk.org/jdk/pull/28566/files/74e72e0b..e10d268d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28566&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28566&range=00-01

  Stats: 109 lines in 1 file changed: 62 ins; 23 del; 24 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