ENH: Close figures in test explicitly

[jhlegarreta]

Close figures in test explicitly: when the output_file argument is None the plot_carpet method does not close figures, and thus a warning is raised if the number of open figures exceeds a given value.

Fixes:

nireports/tests/test_reportlets.py::test_carpetplot[linkage-0.7]
  /home/runner/work/nireports/nireports/nireports/reportlets/nuisance.py:326:
   RuntimeWarning: More than 20 figures have been opened.
   Figures created through the pyplot interface (`matplotlib.pyplot.figure`)
   are retained until explicitly closed and may consume too much memory.
   (To control this warning, see the rcParam `figure.max_open_warning`).
   Consider using `matplotlib.pyplot.close()`.
      figure, allaxes = plt.subplots(figsize=(19.2, 10))

reported for example in:
https://github.com/nipreps/nireports/actions/runs/12681153218/job/35344304375#step:12:351

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.48%. Comparing base (eb3832c) to head (626dd35).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #155      +/-   ##
==========================================
+ Coverage   65.25%   65.48%   +0.22%     
==========================================
  Files          25       25              
  Lines        2674     2674              
  Branches      420      420              
==========================================
+ Hits         1745     1751       +6     
+ Misses        814      810       -4     
+ Partials      115      113       -2     

Flag	Coverage Δ
unittests	64.87% <ø> (?)

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