RFR: 8317042: G1: Make TestG1ConcMarkStepDurationMillis use createTestJvm

[Leo Korinth]

Use createTestJvm.

Tested with: `-XX+UseG1GC` (pass 1), `-XX+UseZGC` (total 0), `-XX:G1ConcMarkStepDurationMillis=23` (total 0) and no options (pass 1)

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Commit messages:
 - 8317042

Changes: https://git.openjdk.org/jdk/pull/15948/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15948&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317042
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/15948.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15948/head:pull/15948

PR: https://git.openjdk.org/jdk/pull/15948