FIX: T1w_preproc -> t1w_preproc

[effigies]

Small fix while writing the smoke tests in #390 turned out to be a typo. The issue was that we weren't using the t1w_buffer as a source for T2w preprocessing, so that if you had a preprocessed T1w image but an unprocessed T2w image, you'd try to grab the T1w from the wrong place. And I typed it wrong, and some of these checks are only found at workflow expansion.

Added workflow expansion to the test to catch this.

Fixes #398.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e46b0e6) 76.94% compared to head (7e2909e) 61.36%.

Additional details and impacted files

@@                Coverage Diff                @@
##           maint/0.13.x     #399       +/-   ##
=================================================
- Coverage         76.94%   61.36%   -15.59%     
=================================================
  Files                28       22        -6     
  Lines              2039     1887      -152     
  Branches            240      240               
=================================================
- Hits               1569     1158      -411     
- Misses              438      701      +263     
+ Partials             32       28        -4     

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