This PR fixes a small bug in the test framework in verify_ecdsa function.
r in ecdsa signature is modulo curve order, so if the point R calculated during verification has x-coordinate that is larger than the curve order, the verification will fail in the test framework but pass in libsecp256k1.
Example (all in hex):
public key: 0289d889551598a0263746c01e5882ccf9b7dc4ca5a37108482c9d80de40e0a8cf
der signature: 3006020104020104 (r = 4, s = 4)
message: 3232323232323232323232323232323232323232323232323232323232323232
libsecp256k1 returns true, test framework returns false.
