Add acceptance tests

[bittner]

We finally add tests that should verify that using this formatter with behave actually works.

Relates to #4

[bittner]

@jenisys Can we directly use the step library of the behave project for these tests, or do we have to do copy+paste?

Examples and related code from the parent project:

• /features/
• /more.features/
• /behave4cmd0/ (step library)

[jenisys]

Currently, the simplest thing is to use a copy of „behave4cmd0“ within this repo. Other alternatives (for now):

• Use behave as git-submodule within this repository (not really nice IMHO)
• Use git subtree to get a copy of „behave4cmd0“ in this repo and still have a link to the original repo (may be slightly better)
• Use git-sparse-checkout, an experimental command in git v2.29 that allows to have a filtered view on the referenced repo)
• Put behave4cmd0 in an own repo and use it as git-submodule in behave and this repo (but there is currently an assumption on its location relative to other stuff hardcoded in „behave4cmd0“; in addition, there is an improved „behave4cmd“ in an own repo, currently not published yet which would need some polishing before it is published)

[bittner]

Tests are miraculously passing, only checks fail (flake8, pylint). I'd rather have the original authors fix that to not destroy the original intent of the code. Hence, merging.

[bittner]

Obviously, we need to add the step implementations to finish this task for real. :zany_face: