Move geneformer test dependency back to requirements-dev.txt

[ivirshup]

This makes the dep available for all CI runs, and makes it available locally for running tests.

cc: @ebezzi @mlin

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.26%. Comparing base (55d0856) to head (53b5572).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1236      +/-   ##
==========================================
+ Coverage   91.18%   91.26%   +0.08%     
==========================================
  Files          80       80              
  Lines        6327     6330       +3     
==========================================
+ Hits         5769     5777       +8     
+ Misses        558      553       -5     

Flag	Coverage Δ
unittests	91.26% <ø> (+0.08%)	⬆️

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