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_impl.h

@@ -27,7 +27,7 @@ SECP256K1_INLINE static int secp256k1_fe_equal(const secp256k1_fe *a, const secp
     SECP256K1_FE_VERIFY(a);
     SECP256K1_FE_VERIFY(b);
     SECP256K1_FE_VERIFY_MAGNITUDE(a, 1);
-    SECP256K1_FE_VERIFY_MAGNITUDE(b, 31);
+    SECP256K1_FE_VERIFY_MAGNITUDE(b, 30);
 
     secp256k1_fe_negate(&na, a, 1);
     secp256k1_fe_add(&na, b);