MRG: defer use of ufunc / memmap test until testing

[matthew-brett]

Avoid doing the ufunc / memmap test until we need it.

Might fix gh-571.

[satra]

this lgtm.

[codecov-io]

Codecov Report

Merging #572 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #572      +/-   ##
==========================================
+ Coverage   94.32%   94.32%   +<.01%     
==========================================
  Files         177      177              
  Lines       24691    24695       +4     
  Branches     2638     2639       +1     
==========================================
+ Hits        23290    23294       +4     
  Misses        925      925              
  Partials      476      476

Impacted Files	Coverage Δ
nibabel/testing/__init__.py	94.79% <100%> (ø)	⬆️
nibabel/testing/np_features.py	100% <100%> (ø)	⬆️
nibabel/tests/test_spatialimages.py	99.7% <100%> (ø)	⬆️
nibabel/tests/test_arrayproxy.py	100% <100%> (ø)	⬆️

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[coveralls]

Coverage increased (+0.0006%) to 96.254% when pulling 30f8ddd on matthew-brett:defer_viral_memmap into b1bf5e7 on nipy:master.

[effigies]

LGTM, too. Anything you want to change before merging, @matthew-brett?

[matthew-brett]

Nothing I want to change - go for it ...