Add basic tests for mixin subverbs

[leander-dsouza]

Basic Info

Info	Result
Primary OS tested on	Ubuntu

Description of contribution in a few bullet points

• This PR adds base integration tests for colcon mixin subverbs, covering the following:

  • Adding mixin repositories
  • Updating mixin repositories
  • Showing available mixins
  • Removing mixin repositories
  • Error handling for invalid inputs and duplicate operations

• Included a couple of mixin files - build-type.mixin and coverage.mixin from the colcon-mixin-repository to serve as a basis for all repository testing.

Description of how this change was tested

• Ensured all the tests pass locally - python3 -m pytest test.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 45.90%. Comparing base (ebedd27) to head (0015daa).

Additional details and impacted files

@@             Coverage Diff             @@
##           master      #54       +/-   ##
===========================================
+ Coverage   15.30%   45.90%   +30.60%     
===========================================
  Files          11       11               
  Lines         562      562               
  Branches       94       94               
===========================================
+ Hits           86      258      +172     
+ Misses        474      282      -192     
- Partials        2       22       +20     

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[cottsay]

Thanks for making some progress on the tests.

Invoking the entirety of colcon as part of the tests here isn't something we'll want to do. There are a variety of reasons, but to name a couple:

1. The developer may have any number of extensions enabled, some of which may interfere with the test expectations.
2. The in-test invocations could cause other extensions to act unpredictably, such as colcon-rerun, which records invocations.

Other extensions test their verbs without using subprocess invocations, and we should do the same here. Examples:

• https://github.com/colcon/colcon-alias/blob/main/test/test_verb_alias.py
• https://github.com/colcon/colcon-core/blob/master/test/test_verb_build.py

[leander-dsouza]

Invoking the entirety of colcon as part of the tests here isn't something we'll want to do.

Thank you for attaching the documentation of the testing pattern of other members of colcon.
I have updated the tests to use CommandContext instead of subprocess to validate each subverb usage.

[cottsay]

Thanks, this is looking a lot better. Looking forward to getting it merged and seeing that coverage number go up.

[cottsay]

We should align the YAML style with the rest of the project. Mind taking a look at #56, which adds a check for this?

[leander-dsouza]

The show module of colcon-mixin does not print the mixins in a standardised YAML or JSON format. Hence, I cannot change this to an expected YAML output.

[cottsay]

Whoa, bummer. That might be worth filing a new issue.

[leander-dsouza]

I have created #58 to ensure that the show subverb would print YAML output.
If you don't mind, could you please take a look at it?

Upon merging the said PR, I will modify demo.txt to have an expected YAML output.

[cottsay]

Unless there's a reason to use JSON here, I think we should use YAML. For one, it has better line-change-cleanliness. The filename can remain the same or change, whatever you prefer.

[leander-dsouza]

I have modified the mixins to use a YAML format instead.
However, yamllint cannot check for linting in these files, as these have to end in a .mixin extension for discovery.

[cottsay]

I merged master and added a commit to remove the special casing around get_config_path now that #55 is merged.