Refactoring tests

[prometherion]

Tests are actually out of control.

Although bats is easy peasy, we're facing several issues with it, mostly regarding the copy/paste required across several test cases.

Furthermore, debugging the suite is tricky and error-prone, due to the limitations of bash.

We should investigate how to improve it, covering all the functionalities in each scenario (kubectl, http, TLS, User, Group, ServiceAccount`, etc.) and covering also #189 to avoid regressions.

[oliverbaehler]

I will close this issue in favor of #257