Tst fix genmod link tests

I got a test failure on TravisCI where required precision was too high
https://github.com/statsmodels/statsmodels/pull/1403#issuecomment-35652928

changes:
use assert_allclose with a mix of rtol and atol  (values can be in a very large range)
convert test class to test_functions

There are no related changes in GEE PR #1314