Add unit tests for ecp_mod_p256k1()

[yanesca]

The task is to add unit tests for ecp_mod_p256k1(). This function reduces the input modulo secp256k1_p. The result of ecp_mod_p256k1() is not always canonical, use mpi_mod_mpi() to get a canonical representative for easier testing.

Only cases where the input is non-negative and at most twice as long as the modulus. (This is because soon these will be replaced by their fixed width _raw counterparts.)

[yanesca]

@minosgalanakis according to our process, the PRs should be on the board and not the issues. Could you please remove this from the board and add the PR instead?