TST: special: test against (nearly) everything in mpmath

This PR improves the test coverage of scipy.special by cross-checking computed values against (nearly) every special function available also in mpmath.

This attempts to cover the whole real/complex-valued parameter range, concentrating on values of order of 1.

Turns out (not surprisingly) that several of the functions originating from Cephes or specfun.f do not pass the muster. These seem largely be due to issues at extremely large or small argument values, issues with rounding |x| < 1e-16 to zero, overflowing to inf somewhat too early, etc etc.

The failing tests are marked as knownfail, which serves as a TODO list.