RFR: 8274521: jdk/jfr/event/gc/detailed/TestGCLockerEvent.java fails when other GC is selected [v2]

[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

Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:

  Proper requires

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5756/files
  - new: https://git.openjdk.java.net/jdk/pull/5756/files/bf70f058..90efd821

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5756&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5756&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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