BUG: fix cpow tests to match what npy_cpow does.

Annex G of the C99 standard does not define any special values for cpow,
allowing the implementation to be as simple as cpow(z,w) =
cexp(w*clog(z)).  We have a large number of tests for our cpow function,
both in the test suite and in test_c99complex.c. (There are actually
more in test_c99complex.c, since I'm doing all combinations from
TestCpow::test_scalar in test_umath_complex.py.)

As of right now, these tests probably mean that we will never use a
system cpow implemenation. This is fine, since we handle a large number
of edge cases in a sane way and at least glibc does not.

With this commit all 48 of our complex functions pass test_c99complex.c.