[tests] add option to run all backends
[tests] add option to run all backends

Note this is an unsupported use case...
Instead run:
```
pytest --seabreeze-backend cseabreeze
pytest --seabreeze-backend pyseabreeze
pytest --seabreeze-backend pyseabreeze:libusb0
pytest --seabreeze-backend pyseabreeze:libusb1
pytest --seabreeze-backend pyseabreeze:openusb
```

ap--/python-seabreeze,ap--/python-seabreeze,ap--/python-seabreeze