flaky test TestTick

[hawkingrei]

Bug Report

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     advancer_test.go:71: 
        	Error Trace:	br/pkg/streamhelper/advancer_test.go:71
        	Error:      	Not equal: 
        	            	expected: 0x5
        	            	actual  : 0x3
        	Test:       	TestTick
>>> fake cluster <<<
regions: 5(1):[, 3031);49L2F0 6(2):[3031, 3032);255L1F0 7(3):[3032, 303232);186L3F0 8(4):[303232, 303233);44L2F0 9(5):[303233, 303333);80L2F0 10(6):[303333, 3034);89L4F0 11(7):[3034, 303433);179L1F0 12(7):[303433, );3L3F0 
1: 12(7):[303433, );3L3F0 5(1):[, 3031);49L2F0 6(2):[3031, 3032);255L1F0 7(3):[3032, 303232);186L3F0 8(4):[303232, 303233);44L2F0 10(6):[303333, 3034);89L4F0 11(7):[3034, 303433);179L1F0 
2: 5(1):[, 3031);49L2F0 6(2):[3031, 3032);255L1F0 8(4):[303232, 303233);44L2F0 9(5):[303233, 303333);80L2F0 10(6):[303333, 3034);89L4F0 11(7):[3034, 303433);179L1F0 
3: 11(7):[3034, 303433);179L1F0 12(7):[303433, );3L3F0 6(2):[3031, 3032);255L1F0 7(3):[3032, 303232);186L3F0 8(4):[303232, 303233);44L2F0 9(5):[303233, 303333);80L2F0 
4: 5(1):[, 3031);49L2F0 7(3):[3032, 303232);186L3F0 9(5):[303233, 303333);80L2F0 10(6):[303333, 3034);89L4F0 12(7):[303433, );3L3F0 
�--- FAIL: TestTick (0.00s) 

https://tiprow.hawkingrei.com/view/gs/pingcapprow/pr-logs/pull/pingcap_tidb/60325/fast_test_tiprow/1906628096010752000#1:build-log.txt%3A1879

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