Add Metal test with few fixed effects

[matthieugomez]

Added a test case for regression with period fixed effects.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.50%. Comparing base (72ca0e0) to head (123a93a).
⚠️ Report is 10 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #275      +/-   ##
==========================================
+ Coverage   96.48%   96.50%   +0.01%     
==========================================
  Files           8        8              
  Lines         655      658       +3     
==========================================
+ Hits          632      635       +3     
  Misses         23       23              

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