Add exhaustive test for group functions on a low-order subgroup

We observe that when changing the b-value in the elliptic curve formula
`y^2 = x^3 + ax + b`, the group law is unchanged. Therefore our functions
for secp256k1 will be correct if and only if they are correct when applied
to the curve defined by `y^2 = x^3 + 4` defined over the same field. This
curve has a point P of order 199.

This commit adds a test which computes the subgroup generated by P and
exhaustively checks that addition of every pair of points gives the correct
result.

Unfortunately we cannot test const-time scalar multiplication by the same
mechanism. The reason is that these ecmult functions both compute a wNAF
representation of the scalar, and this representation is tied to the order
of the group.

Testing with the incomplete version of gej_add_ge (found in 5de4c5dff^)
shows that this detects the incompleteness when adding P - 106P, which
is exactly what we expected since 106 is a cube root of 1 mod 199.

src/field.h

@@ -30,6 +30,8 @@
 #error "Please select field implementation"
 #endif
 
+#include "util.h"
+
 /** Normalize a field element. */
 static void secp256k1_fe_normalize(secp256k1_fe *r);
 
@@ -50,6 +52,9 @@ static int secp256k1_fe_normalizes_to_zero_var(secp256k1_fe *r);
 /** Set a field element equal to a small integer. Resulting field element is normalized. */
 static void secp256k1_fe_set_int(secp256k1_fe *r, int a);
 
+/** Sets a field element equal to zero, initializing all fields. */
+static void secp256k1_fe_clear(secp256k1_fe *a);
+
 /** Verify whether a field element is zero. Requires the input to be normalized. */
 static int secp256k1_fe_is_zero(const secp256k1_fe *a);