RFR: 8314842: zgc/genzgc tests ignore vm flags

[Stefan Karlsson]

Change some ZGC tests to propagate requested vm flags.

I tested this by manually passing in various flags to jtreg. I also ran tier1-5 on Linux.

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Commit messages:
 - 8314842: zgc/genzgc tests ignore vm flags

Changes: https://git.openjdk.org/jdk/pull/20963/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20963&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314842
  Stats: 6 lines in 6 files changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/20963.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20963/head:pull/20963

PR: https://git.openjdk.org/jdk/pull/20963