Add tests for wrapping `c` over $p$

[skaunov]

It might be a RustCrypto thing which just panics on certain methods when bytes yields the value larger than $p$, but it's still a worthy case to have in the suite across implementation to be sure that wrapping is done correctly, and that signatures equality can be facilitated.

[skaunov]

1) add $p$ to the test signature and verify 2) find a hash greater than $p$ 3) test the signing-verifying with it 4) consider adding Eq kind of trait in Rust implementations

(also hash improbable to be equal $p$ and basically should be rejected)