[jdk11u-dev] RFR: 8231943: ZGC: Enable serviceability/dcmd/gc/RunGCTest

[Satyen Subramaniam]

Backporting JDK-8231943: ZGC: Enable serviceability/dcmd/gc/RunGCTest. Adjusting test to be compatible with ZGC by slightly shortening output string check. Also cleaning up test by removing redundant "gc=debug". Ran GHA Sanity Checks, local Tier 1 and 2, and adjusted test directly. Patch is clean.

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Commit messages:
 - Backport db89b5fdf9f1dca06ba569dbf61e15f637a4367b

Changes: https://git.openjdk.org/jdk11u-dev/pull/3060/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=3060&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8231943
  Stats: 4 lines in 1 file changed: 0 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/3060.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/3060/head:pull/3060

PR: https://git.openjdk.org/jdk11u-dev/pull/3060