test: refactor: simplify tests by using _ecmult_gen_ge helper

If the generator point multiplication result in Jacobian coordinates is
immediately converted to affine coordinates after and is not needed for
anything else, we can deduplicate by using the helper introduced in #1861.

Note that in a very strict sense this is not a refactor, as the Jacobian
object is now cleared out which was not done on master, but for the logic
in the tests this shouldn't matter at all.

src/tests_exhaustive.c

@@ -421,12 +421,10 @@ int main(int argc, char** argv) {
             /* Verify against ecmult_gen */
             {
                 secp256k1_scalar scalar_i;
-                secp256k1_gej generatedj;
                 secp256k1_ge generated;
 
                 secp256k1_scalar_set_int(&scalar_i, i);
-                secp256k1_ecmult_gen_gej(&ctx->ecmult_gen_ctx, &generatedj, &scalar_i);
-                secp256k1_ge_set_gej(&generated, &generatedj);
+                secp256k1_ecmult_gen_ge(&ctx->ecmult_gen_ctx, &generated, &scalar_i);
 
                 CHECK(!secp256k1_ge_is_infinity(&group[i]));
                 CHECK(secp256k1_ge_eq_var(&group[i], &generated));