Fix test_sw_properties for some cofactor groups

The sw_properties test failed for groups with cofactors, for which the
first valid point (according to incrementing x) was on the prime order
subgroup.
The test assumed that the first point should have been of low order,
such that multiplying it with the cofactor put it in the high order
group.

This patch simplifies the code, and skips over these points.

Fixes arkworks-rs#553

test-templates/src/groups.rs

@@ -259,16 +259,17 @@ macro_rules! __test_group {
             assert!(generator.is_on_curve());
             assert!(generator.is_in_correct_subgroup_assuming_on_curve());
 
-            let mut x = BaseField::zero();
-            let mut i = 0;
-            loop {
+            for i in 0.. {
+                let x = BaseField::from(i);
                 // y^2 = x^3 + a * x + b
                 let rhs = x * x.square() + x * <Config as SWCurveConfig>::COEFF_A + <Config as SWCurveConfig>::COEFF_B;
 
                 if let Some(y) = rhs.sqrt() {
                     let p = Affine::new_unchecked(x, if y < -y { y } else { -y });
                     if !<<$group as CurveGroup>::Config as CurveConfig>::cofactor_is_one() {
-                        assert!(!p.is_in_correct_subgroup_assuming_on_curve());
+                        if p.is_in_correct_subgroup_assuming_on_curve() {
+                            continue;
+                        }
                     }
 
                     let g1 = p.mul_by_cofactor_to_group();
@@ -278,9 +279,6 @@ macro_rules! __test_group {
                         break;
                     }
                 }
-
-                i += 1;
-                x += BaseField::one();
             }
 
             for _ in 0..ITERATIONS {