RFR: 8326127: JFR: Add SafepointCleanupTask to hardToTestEvents of TestLookForUntestedEvents

[Markus Grönlund]

On Mon, 19 Feb 2024 06:59:02 GMT, Denghui Dong <ddong at openjdk.org> wrote:

> Greetings,
> 
> [JDK-8322630](https://bugs.openjdk.org/browse/JDK-8322630) made the `EventSafepointCleanupTask` hard to test. We'd better move it to hardToTestEvents of TestLookForUntestedEvents instead of commenting out it.
> 
> Denghui

Marked as reviewed by mgronlun (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/17907#pullrequestreview-1888122577