test: Fix bug with TransformedTargetRegressor

[antoinedemathelin]

init of TransformedTargetRegressor have changed

[codecov-commenter]

Codecov Report

Merging #8 (eec5734) into master (926635a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master       #8   +/-   ##
=======================================
  Coverage   89.74%   89.74%           
=======================================
  Files          18       18           
  Lines        1629     1629           
=======================================
  Hits         1462     1462           
  Misses        167      167           

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