Fix TestDefaultResultTracker_StartMinimumRequests_NoFailingRequests flakyness

[pracucci]

What this PR does:

In this CI run we've seen TestDefaultResultTracker_StartMinimumRequests_NoFailingRequests being flaky:

--- FAIL: TestDefaultResultTracker_StartMinimumRequests_NoFailingRequests (20.67s)
    replication_set_tracker_test.go:294: 
        	Error Trace:	/drone/src/ring/replication_set_tracker_test.go:294
        	Error:      	Max difference between 750 and 695 allowed is 50, but difference was 55
        	Test:       	TestDefaultResultTracker_StartMinimumRequests_NoFailingRequests
        	Messages:   	expected roughly even distribution of requests across all instances, but got [{[] 769} {[] 772} {[] 764} {[] 695}]
FAIL

The test checks whether each instance got roughly the same number of requests. The selection of instances getting the "minimum requests" is generated randomly. Since there's no perfect random distribution, the actual difference may be greater than the 50 delta configured for the assertion.

In this PR I've just increased the delta to reduce the likelihood of flaky executions.

Which issue(s) this PR fixes:

N/A

Checklist

• Tests updated
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]