RFR: JDK-8311026: Some G1 specific tests do not set -XX:+UseG1GC

[Matthias Baesken]

Most G1 tests set -XX:+UseG1GC, but a few (e.g. gc/g1/TestVerificationInConcurrentCycle.java) miss that.
This is usually just fine and no problem because G1 is the default anyway.
However in some cases, where a custom JVM changes the default GC, those tests start to fail which is not really necessary.

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Commit messages:
 - JDK-8311026

Changes: https://git.openjdk.org/jdk/pull/14722/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14722&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311026
  Stats: 4 lines in 3 files changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/14722.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14722/head:pull/14722

PR: https://git.openjdk.org/jdk/pull/14722