Testing: Group reusable test data in one file

[contsili]

We group reusable test data as "fixtures" which live in test/fixtures/ . There we can built longitudinal data once and reuse by all future tests, same for a correlation matrix.

eg we could create test/fixtures/longitudinal_fixtures.py

[contsili]

We can also consider putting the fixtures in a conftest.py at test/test_longitudinal/conftest.py. conftest.py removes the need to import the fixtures and is the advice from scientific-python: https://learn.scientific-python.org/development/guides/pytest/

[contsili]

I actually prefer conftest.py for the reason i state in this issue #516 (comment)