[MAINT] refactor tests glm

[Remi-Gau]

• Relates to simplify "complex" tests #3570

Changes proposed in this pull request:

• refactor several complex tests in nilearn/glm/tests
• use more fixtures

[github-actions]

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[codecov]

Codecov Report

Merging #3915 (0b989fa) into main (4e2a954) will increase coverage by 0.08%.
Report is 2 commits behind head on main.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3915      +/-   ##
==========================================
+ Coverage   91.68%   91.77%   +0.08%     
==========================================
  Files         134      134              
  Lines       15750    15750              
  Branches     3283     3283              
==========================================
+ Hits        14441    14454      +13     
+ Misses        765      752      -13     
  Partials      544      544              

Flag	Coverage Δ
macos-latest_3.10	91.68% <ø> (ø)
macos-latest_3.11	91.68% <ø> (?)
macos-latest_3.8	91.64% <ø> (ø)
macos-latest_3.9	91.64% <ø> (ø)
ubuntu-latest_3.10	91.68% <ø> (?)
ubuntu-latest_3.11	91.68% <ø> (ø)
ubuntu-latest_3.8	91.64% <ø> (ø)
ubuntu-latest_3.9	91.64% <ø> (ø)
windows-latest_3.10	91.62% <ø> (ø)
windows-latest_3.11	91.62% <ø> (ø)
windows-latest_3.8	91.59% <ø> (?)
windows-latest_3.9	91.59% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
nilearn/glm/thresholding.py	97.84% <ø> (+2.15%)	⬆️

... and 5 files with indirect coverage changes

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[bthirion]

LGTM so far.

[bthirion]

LGTM, thx !