add examples to test suite

one of the most critical paths, example aren't under scrutiny from the testsuite
this is true for `compas_viewer`, but I'd argue for applying this where possible
in the fast paced world of `compas` for many `examples` are the gateway, hence covering these with pytest is a nice thing to do

```
(compas2) ➜ object git:(main) ✗ py robot.py
Traceback (most recent call last):
File "/Users/jelle/Code/CADCAM/compas_viewer/docs/examples/object/robot.py", line 3, in
from compas_robots.viewer.scene.robotmodelobject import RobotModelObject
ModuleNotFoundError: No module named 'compas_robots.viewer'
```