Fix flaky query-frontend integration tests after #10798

[charleskorn]

What this PR does

Following the changes in #10798, query-frontend integration tests can fail with:

    query_frontend_test.go:375: 
        	Error Trace:	/home/runner/work/mimir/mimir/integration/query_frontend_test.go:375
        	            				/home/runner/work/mimir/mimir/integration/query_frontend_test.go:49
        	Error:      	Received unexpected error:
        	            	unable to find metrics [cortex_frontend_split_queries_total] with expected values. Last error: <nil>. Last values: [32]
        	Test:       	TestQueryFrontendWithBlocksStorageViaFlags

This happens when the test is being run between 00:00 and 01:00 UTC, as then the query touches data in 32 different days and so is split 32 ways.

⚠️ This PR depends on grafana/e2e#20.

Which issue(s) this PR fixes or relates to

#10798

Checklist

• Tests updated.
• [n/a] Documentation added.
• [n/a] CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• [n/a] about-versioning.md updated with experimental features.