storage: fix TestPebbleMetricEventListener

[jbowens]

In #96021 handling of disk slow events was made asynchronous. This introduced a race during Close where the goroutine logging could still be inflight.

Fix #96635.

Epic: None
Release note: None

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[itsbilal]

LGTM

[jbowens]

TFTR!

bors r=itsbilal

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from 27df3cb to blathers/backport-release-22.2-96651: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 22.2.x failed. See errors above.

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