Improve docs to use our fixtures

[maxmeyer]

Summary

This PR improves the documentation about using our fixtures API.

Details

• Fix false positive feature tests - they creating the wrong file and succeed although they should have failed
• Add example about how to use fixtures along with commands

Motivation and Context

I've got an ongoing valuable discussion with @remy about the usage of fixtures. It turned out, that our docs are wrong at some places and additionally a good example was missing to clarify the use of fixtures with commands.

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactoring (cleanup of codebase withouth changing any existing functionality)

Checklist:

• I've added tests for my code
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[aslakhellesoy]

Hi @maxmeyer,
Thanks for your contribution and welcome to the committers team!
You can now push directly to this repo and all other repos under the cucumber
organization. Please read up on the general contrinuting guidelines to learn more.

Cheers,
Aslak Hellesøy
Creator of Cucumber