Use message_ix.testing plugin in "conda" workflow

[khaeru]

This is to address failures in the "conda" CI workflow, e.g. here:

_______________________ ERROR at setup of test_add_cat ________________________
file D:\a\_actions\iiasa\actions\main\setup-conda\Anaconda3\envs\testenv\Lib\site-packages\message_ix\tests\test_core.py, line 376
  def test_add_cat(dantzig_message_scenario: Scenario) -> None:
file D:\a\_actions\iiasa\actions\main\setup-conda\Anaconda3\envs\testenv\Lib\site-packages\message_ix\tests\test_core.py, line 21
  @pytest.fixture
  def dantzig_message_scenario(
E       fixture 'message_test_mp' not found

The fixture in question is in message_ix.testing. When running from source, this plugin is specified by conftest.py, but when installed from PyPI or conda-forge the file is not present and the plugins must be specified directly. (This could possibly be avoided by moving the file to message_ix/tests/conftest.py; we haven't tried this.)

How to review

The "conda" workflow is temporarily enabled for the PR branch.

• Note that the "conda" workflow run passes, here.

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation. N/A; CI changes only
• Update release notes.
• (After approval) Remove the TEMPORARY commit.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.0%. Comparing base (970c3d2) to head (1afd120).
Report is 1 commits behind head on main.

❗ Current head 1afd120 differs from pull request most recent head b30a58c

Please upload reports for the commit b30a58c to get more accurate results.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #944   +/-   ##
=====================================
  Coverage   96.0%   96.0%           
=====================================
  Files         53      53           
  Lines       4998    4998           
=====================================
  Hits        4801    4801           
  Misses       197     197           

[khaeru]

✅ run of "pytest" here. Will now rebase to drop the temporary commit, and merge.