[MAINT] refactor tests datasets

[Remi-Gau]

• Relates to simplify "complex" tests #3570
• Closes dataset fetch errors without test failures? #3911

Changes proposed in this pull request:

• refactor complex tests in nilearn/datasets/tests

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[codecov]

Codecov Report

Merging #3912 (a52eff6) into main (9d5409c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #3912   +/-   ##
=======================================
  Coverage   91.76%   91.76%           
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  Files         134      134           
  Lines       15747    15747           
  Branches     3283     3283           
=======================================
  Hits        14451    14451           
  Misses        752      752           
  Partials      544      544           

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

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[bthirion]

Thx for tackling this !

[Remi-Gau]

• address dataset fetch errors without test failures? #3911

[Remi-Gau]

Sorry for the large number of line changes: a lot of this is due to adding new lines to separate the set up, test, teardown of each test. I find that it improves test readability.

[bthirion]

I think that this is mostly good. Just making sure I understand the diff...

[bthirion]

LGTM, thx !