TST: make test shear matrix full rank

[matthew-brett]

Matrix with shears was rank deficient, so small deviations in the SVD
output were making the tests fail - see:
http://nipy.bic.berkeley.edu/builders/nibabel-py2.7-debian-ppc/builds/106/steps/shell_6/logs/stdio

Make test matrix full rank to avoid test failures.

[coveralls]

Coverage increased (+0.0004%) to 96.008% when pulling ab13027 on matthew-brett:sheared-rank-fix into b8017c9 on nipy:master.

[codecov-io]

Codecov Report

Merging #510 into master will increase coverage by <.01%.

@@            Coverage Diff             @@
##           master     #510      +/-   ##
==========================================
+ Coverage   94.02%   94.03%   +<.01%     
==========================================
  Files         166      166              
  Lines       21992    21994       +2     
  Branches     2343     2343              
==========================================
+ Hits        20679    20681       +2     
  Misses        878      878              
  Partials      435      435

Impacted Files	Coverage Δ
nibabel/tests/test_nifti1.py	98.32% <100%> (ø)	✅

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