musig/tests: initialize keypair

The keypair is unused in musig_partial_sign, but clang-snapshot gives a compiler
warning anyway.

src/modules/musig/tests_impl.h

@@ -1013,6 +1013,8 @@ static void musig_test_vectors_signverify(void) {
         if (!expected) {
             continue;
         }
+        CHECK(secp256k1_ec_pubkey_parse(CTX, &pubkey, vector->pubkeys[0], sizeof(vector->pubkeys[0])));
+        CHECK(secp256k1_keypair_create(CTX, &keypair, vector->sk));
 
         expected = c->error != MUSIG_AGGNONCE;
         CHECK(expected == secp256k1_musig_aggnonce_parse(CTX, &aggnonce, vector->aggnonces[c->aggnonce_index]));