Reorganising pytest fixtures

[sfmig]

Is your feature request related to a problem? Please describe.
We generally prefer fixtures to be close to the test they refer to, but sometimes the files become a bit bloated. It'd be nice for contributors if the tests are easy to read and navigate. For this shorter files are generally nicer.

The goal of this issue is to discuss refactoring options for the fixtures.

Describe the solution you'd like
Some options for refactoring the fixtures are:

• using a conftest.py file(s)
• using pytest_plugins variables like this (we do that in 🦀 s).

Describe alternatives you've considered
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Additional context
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