fix(compactor): Fix flaky TestBlocksCleaner_ShouldRemoveBlocksOutside…

[yeya24]

…RetentionPeriod

Switch from filesystem bucket to in-memory bucket in this test. The filesystem bucket's Upload is non-atomic (os.Create truncates, then io.Copy writes), creating a race window where the HeartBeat goroutine's write can be observed as an empty file by a concurrent read. The in-memory bucket's Upload is atomic (mutex-protected), eliminating the race.

What this PR does:

Fix flaky test by switching to use an inmemory bucket to eliminate possible race condition when the visit marker is being uploaded.

--- FAIL: TestBlocksCleaner_ShouldRemoveBlocksOutsideRetentionPeriod (0.06s)
    blocks_cleaner_test.go:822: 
        	Error Trace:	/__w/cortex/cortex/pkg/compactor/blocks_cleaner_test.go:822
        	Error:      	Received unexpected error:
        	            	failed to read cleaner visit marker: visit marker file: markers/cleaner-visit-marker.json, content: , error: unexpected end of JSON input: unmarshal visit marker JSON
        	Test:       	TestBlocksCleaner_ShouldRemoveBlocksOutsideRetentionPeriod

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]
• docs/configuration/v1-guarantees.md updated if this PR introduces experimental flags

[SungJin1212]

thanks

[friedrichg]

Thanks!