Simplify conda use in tests

[dstansby]

I noticed some redundant conda commands - conda-incubator/setup-miniconda should activate the right environment at each step, and make sure the right python is installed.

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/tutorial.rst
• Changes documented in docs/release.rst
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.99%. Comparing base (6105ef2) to head (5544486).

❗ Current head 5544486 differs from pull request most recent head d3457cd. Consider uploading reports for the commit d3457cd to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1715   +/-   ##
=======================================
  Coverage   99.98%   99.99%           
=======================================
  Files          38       38           
  Lines       14640    14580   -60     
=======================================
- Hits        14638    14579   -59     
+ Misses          2        1    -1     

see 7 files with indirect coverage changes

[jhamman]

Thanks for this @dstansby - as you now know, we're onto the v3 branch where we're not using conda, so I'm going to close this out.