Add test for secp256k1_fe_half

Author: sipa

src/tests.c

@@ -2548,6 +2548,14 @@ void run_field_misc(void) {
         secp256k1_fe_add(&q, &x);
         CHECK(check_fe_equal(&y, &z));
         CHECK(check_fe_equal(&q, &y));
+        /* Check secp256k1_fe_half. */
+        z = x;
+        secp256k1_fe_half(&z);
+        secp256k1_fe_add(&z, &z);
+        CHECK(check_fe_equal(&x, &z));
+        secp256k1_fe_add(&z, &z);
+        secp256k1_fe_half(&z);
+        CHECK(check_fe_equal(&x, &z));
     }
 }