Fix "build status" aka tests badge in README

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (865af43) 100.00% compared to head (2c1c298) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #634 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 4 4 Lines 40 40 Branches 5 5 ========================================= Hits 40 40 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.

[ChristophWurst]

too slow