TST: bump test tolerance for linalg.orth test a little.

Fixes failure observed on OS X.
TST: fix test errors in sparse tests that compare NaNs.  Closes gh-3901.

Note that only __gt__ currently raises a warning, __lt__ doesn't. This is an
inconsistency in numpy it looks like.
Merge pull request #3910 from rgommers/test-errors

BUG: Fix a couple of test errors in master