tracing_serial_test fails due to wrong time expectation

[martin-g]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Run make br_unit_test

2. What did you expect to see? (Required)

All tests pass.

3. What did you see instead (Required)

Detail BR trace in /tmp/TestSpan3186194723/001/br.trace 
[2021/12/06 15:36:36.914 +02:00] [INFO] [tracing.go:59] ["Detail BR trace"] [filename=/tmp/TestSpan3186194723/001/br.trace]
--- FAIL: TestSpan (0.03s)
    tracing_serial_test.go:51: 
        	Error Trace:	tracing_serial_test.go:51
        	Error:      	Expect "jobA      15:36:36.882226  31.770979ms
        	            	  └─jobB  15:36:36.882227  19.032306ms
        	            	" to match "^jobA.*2[0-9]\.[0-9]+ms\n  └─jobB.*1[0-9]\.[0-9]+ms\n$"
        	Test:       	TestSpan
FAIL
FAIL	github.com/pingcap/tidb/br/pkg/trace	0.040s
ok  	github.com/pingcap/tidb/br/pkg/utils	12.129s

4. What is your TiDB version? (Required)

v4.0.0-beta.2-3094-g6900a4622

[3pointer]

#30716 fixed it.

[github-actions]

Please check whether the issue should be labeled with 'affects-x.y' or 'fixes-x.y.z', and then remove 'needs-more-info' label.

[martin-g]

Confirmed! The test passes now!