Convert the test suite to 100% pytest-based

[jaimergp]

Checklist

• I added a descriptive title
• I searched open requests and couldn't find a duplicate

What is the idea?

Our current test infrastructure uses some pytest logic, but the main logic happens in the run_examples.py script, which depends on the content of examples/. This is a homemade, ad-hoc script that has grown a bit too much, with many features and cases. Every time we add a new variable to check it's shoehorned in and it is not getting any better.

I suggest we use a pytest-only test infra, powered by fixtures and other pytest goodies. The good folks at MNE (@larsoner more specifically) did suggest something along these lines a while ago (see comment here for more details and ideas).

This would allow us to test better, clearer and more efficiently. Debugging should also get easier thanks to the -k flag.

Why is this needed?

• Better development experience
• Long-term maintainability
• Readability of the test infra

What should happen?

• All the examples should be covered by a pytest case, with a docstring explaining what we are aiming for in that test.
• Post-installation checks wouldn't need the post_install.sh hacks, and could be done with pytest idioms. Testing with Bash makes me feel bad :D

Additional Context

No response

[goanpeca]

I can also help with this! :)