Improve pytest warning on missing test data

[Schamper]

This makes it clearer when test data is not properly available.

❯ uv run pytest
============================= test session starts ==============================
platform darwin -- Python 3.11.12, pytest-8.3.5, pluggy-1.5.0
configfile: pyproject.toml
collected 0 items

! ! !
Test data files look like git-lfs references.
This can happen when git-lfs is not installed or not configured correctly.
Install git-lfs and run:
git lfs install && git lfs pull
! ! !
============================ no tests ran in 0.00s =============================

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.84%. Comparing base (93f91ce) to head (b7dc5d6).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1119   +/-   ##
=======================================
  Coverage   77.84%   77.84%           
=======================================
  Files         357      357           
  Lines       32367    32367           
=======================================
  Hits        25196    25196           
  Misses       7171     7171           

Flag	Coverage Δ
unittests	77.84% <ø> (ø)

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