test(cli): convert CLI shell tests to pytest test cases

[nejch]

An initial attempt at unifying the testing approach 😁

This PR uses the pytest-console-scripts plugin and provides a gitlab_cli() wrapper for tests, so people can add gitlab CL subcommands as a list and include variables (from the test case or from fixtures) to assert against.

I'm unsure if this is really the best approach, but I've already been impressed while building this by how much more detail pytest provides for each error/failure, even when running CLI tests. If I did the fixtures right, they should also later be reusable for API functional tests. :P

For now I've not split the tests yet, to make reviews easier (1 shell test -> 1 pytest test), which makes the file quite long. I guess it would also make sense to split them. This would be better anyway, and this should make tests not depend on each other (I'm almost there even now - still have a few todos for some fixtures).

If this approach makes sense I can add some docs on how to contribute tests to make it more accessible for new contributors, :)

An example run is here: https://travis-ci.org/github/python-gitlab/python-gitlab/jobs/679739257

Edit: I've also not yet put this inside gitlab/tests yet, to avoid hacking the tox scripts, but I guess the structure will also be a topic

[codecov-io]

Codecov Report

Merging #1086 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1086   +/-   ##
=======================================
  Coverage   76.68%   76.68%           
=======================================
  Files          10       10           
  Lines        2758     2758           
=======================================
  Hits         2115     2115           
  Misses        643      643           

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