Add basic usage testcases

[zeisss]

In #79 we learned that we forget certain cases. Just defining a testsuite to run through the user journey and verifiying all commands could help mitigate this in the future.

Years ago I used something called cram tests to verify command line tool outputs, maybe this could be used here? But its looks rather inactive, maybe there is something more modern.

• https://bitheap.org/cram/
• https://github.com/giantswarm/cli/blob/master/Makefile#L155
• https://github.com/giantswarm/cli/tree/master/int-tests

[zeisss]

See https://mmap.page/cli/test/ for a listing of various tools. Cram still looks the best, although rather dead :(