`secp256k1` additional test vectors

[skaunov]

There was an implementation of secp256k1 curve (now deprecated) which beside of the common arkworks test templates use the specific values. I wonder if those should be added to the current curve crate in the repo as well?
https://github.com/geometryxyz/ark-secp256k1/blob/d508c22e4b24d13b9a13f89320c59ba62d22c529/src/test_vectors.rs#L8

The test which uses it and the additional serialization test (is it useful too?) are at https://github.com/geometryxyz/ark-secp256k1/blob/d508c22e4b24d13b9a13f89320c59ba62d22c529/src/tests.rs#L87.

[Pratyush]

These would be welcome; would you care to make a PR?