Add first party support for k256 and p256 under feature flags

Given the lack of vectors for k256, it's currently a match of the p256 
spec (with a distinct context string), yet p256 is still always used 
when testing.

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

@@ -1 +1,2 @@
-mod p256;
+mod expand_message;
+mod kp256;