Generalize out the FROST test for signing/signing with an offset

Moves Schnorr signature tests from test_curve to the new test_schnorr, 
which is more a test_frost.

Relevant to https://github.com/serai-dex/serai/issues/9.

crypto/frost/src/tests/literal/mod.rs

@@ -1,2 +1 @@
 mod p256;
-mod schnorr;