Fix flaky tests in result caches

[SungJin1212]

This PR fixes the flakiness of the TestResultsCachePutTTLSelection and TestExtentsOverlapOutOfOrderWindow by injecting the now function.

Related Test Fail Log

--- FAIL: TestExtentsOverlapOutOfOrderWindow (0.00s)
    --- FAIL: TestExtentsOverlapOutOfOrderWindow/extent_ends_exactly_at_cutoff_boundary_-_overlaps_(boundary_case) (0.00s)
        results_cache_test.go:1860: 
            	Error Trace:	/__w/cortex/cortex/pkg/querier/tripperware/queryrange/results_cache_test.go:1860
            	Error:      	Not equal: 
            	            	expected: true
            	            	actual  : false
            	Test:       	TestExtentsOverlapOutOfOrderWindow/extent_ends_exactly_at_cutoff_boundary_-_overlaps_(boundary_case)
FAIL

--- FAIL: TestResultsCachePutTTLSelection (0.00s)
    --- FAIL: TestResultsCachePutTTLSelection/recent_data_uses_out_of_order_results_cache_ttl (0.00s)
        results_cache_test.go:1951: 
            	Error Trace:	/__w/cortex/cortex/pkg/querier/tripperware/queryrange/results_cache_test.go:1951
            	Error:      	Not equal: 
            	            	expected: 5m0s
            	            	actual  : 24h0m0s
            	Test:       	TestResultsCachePutTTLSelection/recent_data_uses_out_of_order_results_cache_ttl
FAIL

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]

Nice thanks for the fix!