Flaky TestSweepImmediateDropsSamples

[pracucci]

While running the CI (see here) for a PR doing changes unrelated to flushing, I've noticed that TestSweepImmediateDropsSamples is flaky.

The test has been recently introduced in #2778.

[pracucci]

I'm reopening the issue because this test is still flaky (see here):

--- FAIL: TestSweepImmediateDropsSamples (0.01s)
    flush_test.go:97: 
        	Error Trace:	flush_test.go:97
        	            				flush_test.go:41
        	            				flush.go:354
        	            				flush.go:319
        	            				flush.go:220
        	            				asm_amd64.s:1373
        	Error:      	Received unexpected error:
        	            	rpc error: code = Code(500) desc = user=1: ingester stopping
        	Test:       	TestSweepImmediateDropsSamples
    flush_test.go:52: 
        	Error Trace:	flush_test.go:52
        	Error:      	Not equal: 
        	            	expected: 2
        	            	actual  : 0
        	Test:       	TestSweepImmediateDropsSamples

[pstibrany]

I think that PR is based on older master (54f10c2) without the fix.