Remove duplicated test in stats
Remove duplicated test in wcs
Add back test_biweight_midvariance_small test.

As pointed out by @astrofrog, this test was not a duplicate
of test_biweight_location_small, just the function name
was duplicated due to a copy & paste error.
Merge pull request #1569 from cdeil/duplicated_signatures

Remove duplicated signatures of in `stats` and `wcs` tests