TEST: Multi-version BBRegister test

[effigies]

Added test_BBRegister.py (based on interfaces.fsl.tests.test_FILMGLS) to stop make specs from building an auto-test. Contains auto-test input maps for 5.3 and 6, and runs against whichever spec is loaded (6 by default, 5.3 if 5.3 is installed).

Also added an explicit test of BBRegister, which caught that init_reg_file ought to be set as non-mandatory in FS 6+.

Closes #1883.

cc @mgxd

[codecov-io]

Codecov Report

Merging #1884 into master will increase coverage by <.01%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master    #1884      +/-   ##
==========================================
+ Coverage   73.11%   73.11%   +<.01%     
==========================================
  Files        1061     1061              
  Lines       53375    53400      +25     
==========================================
+ Hits        39025    39046      +21     
- Misses      14350    14354       +4

Flag	Coverage Δ
#unittests	73.11% <87.5%> (ø)	✅

Impacted Files	Coverage Δ
nipype/interfaces/freesurfer/preprocess.py	69.04% <100%> (+0.03%)	✅
...ype/interfaces/freesurfer/tests/test_preprocess.py	93.15% <85.71%> (-3.08%)	❌
...ype/interfaces/freesurfer/tests/test_BBRegister.py	88.88% <88.88%> (ø)

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