field: correct fe_equal's b magnitude bound

`secp256k1_fe_equal` negates `a` before adding `b`.
That gives the temporary value magnitude 2, and the following field addition requires the input magnitudes to sum to at most 32.
So the largest `b` magnitude the implementation can accept is 30, not 31.

Lower the documented and checked bound for `b` to 30.
Adjust the focused test to use random field elements with randomized magnitudes within the accepted `a <= 1` and `b <= 30` bounds.

Co-authored-by: Sebastian Falbesoner <sebastian.falbesoner@gmail.com>
Co-authored-by: Tim Ruffing <me@real-or-random.org>

src/field.h

@@ -166,7 +166,7 @@ static int secp256k1_fe_is_odd(const secp256k1_fe *a);
 /** Determine whether two field elements are equal.
  *
  * On input, a and b must be valid field elements with magnitudes not exceeding
- * 1 and 31, respectively.
+ * 1 and 30, respectively.
  * Returns a = b (mod p).
  */
 static int secp256k1_fe_equal(const secp256k1_fe *a, const secp256k1_fe *b);