Switched to github.com/stretchr/testify in tests

[codecov[bot]]

Codecov Report

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

Comparison is base (85f53c6) 88.23% compared to head (73c4590) 88.23%. Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #391 +/- ## ======================================= Coverage 88.23% 88.23% ======================================= Files 4 4 Lines 595 595 ======================================= Hits 525 525 Misses 53 53 Partials 17 17 ``` | [Flag](https://app.codecov.io/gh/aerokube/ggr/pull/391/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aerokube) | Coverage Δ | | |---|---|---| | [go](https://app.codecov.io/gh/aerokube/ggr/pull/391/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aerokube) | `88.23% <100.00%> (ø)` | | 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=aerokube#carryforward-flags-in-the-pull-request-comment) to find out more.

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