Refactor github/commitstatus tests

[marco-m-pix4d]

Since to address the bug found by @aliculPix4D in #128 I want to add a repro test, I am doing this pure refactoring PR first, to reduce spurious diffs.

• test: github/commitstatus: refactor: use gotest.tools. We are already importing the gotest.tools package in cogito, but this test file was not yet using it.
• test: github/commitstatus: refactor: use same testing pattern. I found the testing pattern for table-driven tests suggested by the gotest.tools author, https://github.com/gotestyourself/gotest.tools/wiki/Go-Testing-Patterns#table-driven-tests, to be way more readable than that standard approach.

[aliculPix4D]

So I think that you either did a mistake in the first commit or if that was intended (personally I don't think so) there is more cleanup to do.