TST: Reduce sensitivity on these tests.

This level of accuracy is not handled well on all platforms:

http://nipy.bic.berkeley.edu/builders/dipy-bdist32-27/builds/168/steps/shell_8/logs/stdio

http://nipy.bic.berkeley.edu/builders/dipy-py2.7-win32/builds/147/steps/shell_6/logs/stdio