TEST: Do not modify SUBJECTS_DIR contents

[effigies]

Fixes #2131.

Changes proposed in this pull request

• Do not run MRIsExpand interface, to avoid touching $SUBJECTS_DIR/fsaverage/surf.

Loses the verification that the command line is untouched by running. We could restore it by making a copy of the fsaverage directory instead, if it seems worth it.

[satra]

@effigies - could this be merged with master?

[codecov-io]

Codecov Report

Merging #2133 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2133      +/-   ##
==========================================
- Coverage   72.28%   72.28%   -0.01%     
==========================================
  Files        1171     1171              
  Lines       58622    58619       -3     
  Branches     8433     8433              
==========================================
- Hits        42377    42374       -3     
  Misses      14887    14887              
  Partials     1358     1358

Flag	Coverage Δ
#smoketests	72.28% <100%> (-0.01%)	⬇️
#unittests	69.95% <100%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
nipype/interfaces/freesurfer/tests/test_utils.py	96.77% <100%> (-0.11%)	⬇️

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