TestConstantArrivalRateRunCorrectRate is flaky

[na--]

From https://github.com/loadimpact/k6/runs/1385215874?check_suite_focus=true:

 --- FAIL: TestConstantArrivalRateRunCorrectRate (5.05s)
    assertions.go:247: 
                          
	Error Trace:	constant_arrival_rate_test.go:119
                   	
			asm_amd64.s:1373
        
	Error:      	Max difference between 50 and 48 allowed is 1, but difference was 2
        
	Test:       	TestConstantArrivalRateRunCorrectRate

[na--]

From https://github.com/loadimpact/k6/pull/1849/checks?check_run_id=1870090415 (#1849)

 --- FAIL: TestConstantArrivalRateRunCorrectRate (5.01s)
    assertions.go:247: 
                          
	Error Trace:	constant_arrival_rate_test.go:119
        
	            	
			asm_amd64.s:1373
        
	Error:      	Max difference between 50 and 52 allowed is 1, but difference was -2
        
	Test:       	TestConstantArrivalRateRunCorrectRate