RFR: 8274521: jdk/jfr/event/gc/detailed/TestGCLockerEvent.java fails when other GC is selected

[Aleksey Shipilev]

Simple test bug, need to check if G1 is enabled, or it is a default GC before asking `othervm` with `-XX:+UseG1GC`. Other tests in the same directory do exactly that.

Additional testing:
 - [x] Affected test skipped with Shenandoah now
 - [x] Affected test still passes with G1
 - [x] Affected test still passes default GC

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Commit messages:
 - Fix

Changes: https://git.openjdk.java.net/jdk/pull/5756/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5756&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274521
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5756.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5756/head:pull/5756

PR: https://git.openjdk.java.net/jdk/pull/5756