fix flaky TestRequestBuffers

[ortuman]

The test makes use of a sync.Pool under the hood, which apparently does not consistently ensure that when requesting a new item, the item returned is always the one that was previously returned to the pool.

To avoid this effect, it has been decided to revert to using a type fakePool, as was done previously before the #8082 change was introduced.

What this PR does

To validate that the test is no longer flaky, the following command has been executed:

go test -v ./pkg/util -run TestRequestBuffers -count 1000000 -failfast -timeout 60m

Which issue(s) this PR fixes or relates to

Fixes N/A

ref: https://github.com/grafana/mimir/actions/runs/9057849698/job/24882485441?pr=8114#step:8:79

[pstibrany]

which apparently does not consistently ensure that when requesting a new item, the item returned is always the one that was previously returned to the pool.

It can't guarantee that, as items from the pool can also be garbage-collected.