RFR: 8342285: Mark jdk/jfr/api/event/TestIsEnabledMultiple.java as intermittent

[SendaoYan]

Hi all,
The test `jdk/jfr/api/event/TestIsEnabledMultiple.java` has been observed intermittent failure several times. I think we should mark this test as intermittent failure. Trivial fix, no risk.

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Commit messages:
 - 8342285: Mark jdk/jfr/api/event/TestIsEnabledMultiple.java as intermittent

Changes: https://git.openjdk.org/jdk/pull/21666/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21666&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342285
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/21666.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21666/head:pull/21666

PR: https://git.openjdk.org/jdk/pull/21666