TST: stats: Fix some spurious warnings generated when the stats unit tests are run.

When given a negative argument, special.bdtr and special.pdtr return nan, and using
a nan value in a numpy comparison generates a warning.  This commit ensures that
the values used in the comparison that takes place in binom._ppf and poisson._ppf are
not nan.
Merge pull request #3482 from WarrenWeckesser/discr-warnings

TST: stats: Fix some spurious warnings generated when the stats unit tests are run.