RFR: 8315051: jdk/jfr/jvm/TestGetEventWriter.java fails with non-JVMCI GCs

[Aleksey Shipilev]

See the bug for example test failure. There are two ways to fix it: bring in `Whitebox` and sense the JVMCI support from there, or use the `@requires` to sense this and other conditions for JVMCI enablement then. `@requires` seems safer, so I did that one.

Additional testing:
 - [x] Affected test with `-XX:+UseShenandoahGC` (now passes)
 - [x] Affected test with `-XX:+UseParallelGC` (still passes)

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/15446/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15446&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315051
  Stats: 28 lines in 1 file changed: 24 ins; 3 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/15446.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15446/head:pull/15446

PR: https://git.openjdk.org/jdk/pull/15446