Add test coverage for cofactor ECDH using NIST test vectors

Add test coverage for cofactor ECDH using NIST test vectors

Since all non-binary NIST curves have cofactor 1, this is in effect plain
ECDH. Current regress coverage of ECDH is quite lacking on architectures
where Go isn't available. This fixes that.

Actual cofactor ECDH support may be added soon to libcrypto, at which
point I will also add testcases with cofactor > 1.