tidb repo codecov: no need to upload *_test.go coverage

[JasonWu0506]

Bug Report

Please answer these questions before submitting your issue. Thanks!
take an example

1. Minimal reproduce step (Required)

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

no code coverage of *_test.go data uploaded

3. What did you see instead (Required)

code coverage of *_test.go data uploaded as screenshot said

4. What is your TiDB version? (Required)

master

[JasonWu0506]

/assign @hawkingrei