Cellfinder workflow tests

[sfmig]

A first approach to defining tests for the cellfinder workflow.

We eventually didn't pursue the environment variable approach to define the input config, so closing this PR for now. The workflow with tests is now part of PR #23

[codecov]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (8665e74) 0.00% compared to head (3af3b8a) 88.23%.
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##            main      #20       +/-   ##
==========================================
+ Coverage   0.00%   88.23%   +88.23%     
==========================================
  Files          1        2        +1     
  Lines          5      102       +97     
==========================================
+ Hits           0       90       +90     
- Misses         5       12        +7     

Files	Coverage Δ
brainglobe_workflows/__init__.py	60.00% <60.00%> (ø)
brainglobe_workflows/cellfinder/cellfinder_main.py	89.69% <89.69%> (ø)

... and 1 file with indirect coverage changes

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