houghLinesP test should include comments of x1, x2, y1, y2

This is the correct mapping from a Vec4 to x1, x2, y1, y2.
```
const y2 = line.x;
const y1 = line.z;
const x1 = line.y;
const x2 = line.w;
```
Perhaps this can be added to the test-case to document the return value better.