• Fix test_constant_wnaf for -1 and add a test for it.

Before, test_constant_wnaf used scalar_cadd_bit to correct for the skew. But
this function does not correctly deal with overflows which is why num = -1
couldn't be tested.

This commit also adds tests for 0, 1/2 and 1/2-1 as they are corner cases
in constant_wnaf.

• Remove unnecessary sign variable from wnaf_const

This is a backport of libsecp256k1 PR741