reduce test data size and relax test timouts

[alessandrofelder]

Description

What is this PR

• Bug fix
• Addition of a new feature
• Other

Why is this PR needed?

Tests were timing out weirdly, and spuriously on CI only, presumably because of connectivity issues.

What does this PR do?
Simplifies the tests by removing the fish atlas (which was there because we wanted tests for additional-reference related functionality), and implementing a manual mocking an additional reference for the example mouse atlas instead.

References

Closes #158

How has this PR been tested?

Makes CI reliably pass again from what I can see.

Checklist:

• The code has been tested locally
• Tests have been added to cover all new functionality (unit & integration)
• The documentation has been updated to reflect any changes
• The code has been formatted with pre-commit

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.75%. Comparing base (059d669) to head (a0e50b9).
Report is 1 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (059d669) and HEAD (a0e50b9). Click for more details.

HEAD has 2 uploads less than BASE

Flag	BASE (059d669)	HEAD (a0e50b9)
	8	6

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #162       +/-   ##
===========================================
- Coverage   98.06%   82.75%   -15.31%     
===========================================
  Files          11       12        +1     
  Lines         465      551       +86     
===========================================
  Hits          456      456               
- Misses          9       95       +86     

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[alessandrofelder]

Now that everything is cached again, tests finally pass 🎉