WIP: Visuals tests

Currently basic tests for different types of polygons, ellipses and arcs have been added.
And there is no reason to make the visual a property of the `TestingCanvas` as:
```
c.polygon = visuals.Polygon(pos=pos, color=(1, 0, 0, 1))
```
It is cleaner to just do:
```
polygon = visuals.Polygon(pos=pos, color=(1, 0, 0, 1))
```