Fix flaky TestPerLabelsetDiscardedSeriesTracker

[SungJin1212]

Fix flaky test TestPerLabelsetDiscardedSeriesTracker.

How to reproduce

go test -count=100 -race -run TestPerLabelsetDiscardedSeriesTracker ./pkg/util/discardedseries

--- FAIL: TestPerLabelsetDiscardedSeriesTracker (0.00s)
    perlabelset_tracker_test.go:117: 
        	Error Trace:	/__w/cortex/cortex/pkg/util/discardedseries/perlabelset_tracker_test.go:117
        	            				/__w/cortex/cortex/pkg/util/discardedseries/perlabelset_tracker_test.go:84
        	Error:      	Not equal: 
        	            	expected: 0
        	            	actual  : 1
        	Test:       	TestPerLabelsetDiscardedSeriesTracker
FAIL

The reason is that all users share labelsetsToDelete. When processing user2, the labelSetId for user1 could be deleted.
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]

[yeya24]

Thanks for fixing!