test: add xfail for 3d transform on 4d data

[oesteban]

Summary

• add regression test for applying a single 3D transform to a 4D dataset

Testing

• pytest nitransforms/tests/test_resampling.py::test_apply_single_3d_on_4d -q

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https://chatgpt.com/codex/tasks/task_e_689f54b6c3bc8330a3230878ab5c4454

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 96.74%. Comparing base (0c2ad9f) to head (1428106).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #282   +/-   ##
=======================================
  Coverage   96.74%   96.74%           
=======================================
  Files          16       16           
  Lines        1994     1994           
  Branches      267      267           
=======================================
  Hits         1929     1929           
  Misses         41       41           
  Partials       24       24           

Flag	Coverage Δ
unittests	96.72% <ø> (ø)

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