RFR: 8343754: Problemlist jdk/jfr/event/oldobject/TestShenandoah.java after JDK-8279016

[Aleksey Shipilev]

See comment in [JDK-8279016](https://bugs.openjdk.org/browse/JDK-8279016). I overlooked the case when we just run with Shenandoah without explicitly specifying -XX:+UseShenandoahGC. The test should be disabled in that mode as well.

Additional testing:
 - [x] Affected test is now skipped
 - [x] `jdk_jfr` out of the box, now passes
 - [x] `jdk_jfr` with `-XX:+UseShenandoahGC`, still passes

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Commit messages:
 - Also problemlist a generic test

Changes: https://git.openjdk.org/jdk/pull/21947/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21947&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343754
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/21947.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21947/head:pull/21947

PR: https://git.openjdk.org/jdk/pull/21947