Fix the ci for pull-requests
Fix the ci for pull-requests

- Run tests as script (with race condition)
- Put `goverwalls` as `after_script` so a failure to upload the
  coverage won't affect the status of the build

Signed-off-by: Vincent Demeester <87e1f221a672a14a323e57bb65eaea19d3ed3804@sbr.pm>

imdario/mergo