Tests: add coverage config

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (469afbb) 75.60% compared to head (bd25402) 35.22%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #82 +/- ## =========================================== - Coverage 75.60% 35.22% -40.39% =========================================== Files 541 2414 +1873 Lines 59482 124838 +65356 =========================================== - Hits 44974 43975 -999 - Misses 14508 80863 +66355 ``` | [Flag](https://app.codecov.io/gh/yanyongyu/githubkit/pull/82/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Ju4tCode) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/yanyongyu/githubkit/pull/82/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Ju4tCode) | `35.22% <100.00%> (-40.39%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Ju4tCode#carryforward-flags-in-the-pull-request-comment) to find out more.

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