Change exhaustive test groups so they have a point with X=1

This enables testing overflow is correctly encoded in the recid, and
likely triggers more edge cases.

Also introduce a Sage script to generate the parameters.

src/scalar_impl.h

@@ -253,6 +253,7 @@ static void secp256k1_scalar_inverse_var(secp256k1_scalar *r, const secp256k1_sc
 }
 
 #ifdef USE_ENDOMORPHISM
+/* These parameters are generated using sage/gen_exhaustive_groups.sage. */
 #if defined(EXHAUSTIVE_TEST_ORDER)
 #  if EXHAUSTIVE_TEST_ORDER == 13
 #    define EXHAUSTIVE_TEST_LAMBDA 9