testsc: fix memory leak in test_ntru.

We forgot to free the key pair at the end of the test, which is
harmless except that it makes Leak Sanitiser complain loudly.

test/testsc.c

@@ -1610,6 +1610,8 @@ static void test_ntru(void)
                 ntru_encode_plaintext(plaintext, p, BinarySink_UPCAST(buffer));
                 log_end();
 
+                ntru_keypair_free(keypair);
+
                 break;
             }