More test cases for the secp256k1 public key and private key

[sorpaas]

We need more test cases. They can be added as additional files in the tests folder. Both the go implementation and C implementation have good test coverage and many can be ported to this library. This include:

• https://github.com/btcsuite/btcd/blob/master/btcec/privkey_test.go
• https://github.com/btcsuite/btcd/blob/master/btcec/pubkey_test.go
• https://github.com/btcsuite/btcd/blob/master/btcec/signature_test.go
• https://github.com/bitcoin/secp256k1/blob/master/src/tests.c