[MAINT] improve test coverage BaseMasker

[Remi-Gau]

• closes none

• relates to [MAINT] refactor nilearn/maskers (sourcery, f-strings) #3685

• add test to improve test coverage of BaseMasker.fit_transform

• is only meant to check that the test pass on main before refactoring, to ensure behavior consistency in [MAINT] refactor nilearn/maskers (sourcery, f-strings) #3685

[github-actions]

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[codecov]

Codecov Report

Merging #3721 (c6a69f7) into main (b576d9f) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3721      +/-   ##
==========================================
+ Coverage   91.58%   91.60%   +0.02%     
==========================================
  Files         139      139              
  Lines       16596    16596              
  Branches     3243     3243              
==========================================
+ Hits        15199    15203       +4     
+ Misses       1394     1390       -4     
  Partials        3        3              

see 1 file with indirect coverage changes

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[ymzayek]

Great thanks!

[Remi-Gau]

OK will merge and then rebase #3685