Speed-up samples processed from cache stats integration tests (#11949)

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#### What this PR does
This PR speeds up the integration tests for "samples processed from
cache" stats by starting the Mimir cluster once per test, instead of
restarting it for every test case.

On my laptop, the test duration went from 144 seconds to 30.

#### Checklist

- [N/A] Tests updated.
- [N/A] Documentation added.
- [N/A] `CHANGELOG.md` updated - the order of entries should be
`[CHANGE]`, `[FEATURE]`, `[ENHANCEMENT]`, `[BUGFIX]`. If changelog entry
is not needed, please add the `changelog-not-needed` label to the PR.
- [N/A]
[`about-versioning.md`](https://github.com/grafana/mimir/blob/main/docs/sources/mimir/configure/about-versioning.md)
updated with experimental features.

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Signed-off-by: Innokentii Konstantinov <innokenty.konstantinov@grafana.com>

Author: Innokentii Konstantinov