Flaky test TestIngester_flushing

[pracucci]

Describe the bug
The recently introduced test TestIngester_flushing/flushMultipleBlocksWithDataSpanning3Days looks to be flaky:

--- FAIL: TestIngester_flushing (2.19s)
    --- FAIL: TestIngester_flushing/flushMultipleBlocksWithDataSpanning3Days (0.84s)
        ingester_v2_test.go:1902: 
            	Error Trace:	ingester_v2_test.go:1902
            	            				ingester_v2_test.go:1950
            	Error:      	Not equal: 
            	            	expected: 1
            	            	actual  : 0
            	Test:       	TestIngester_flushing/flushMultipleBlocksWithDataSpanning3Days
            	Messages:   	Expected number of calls (1) does not match the actual number of calls (0).

To Reproduce
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