HITL: If we are using the 'null' toolkit then mock the SVGButton class

Bit of a hack, submitted an issue to enable to get SVGButton working with the null toolkit

https://github.com/enthought/enable/issues/162
HITL: Use null toolkit for print_device_details.py

Otherwise imports from settings_view.py will complain that there is no X windows

Also, some cleanup
HITL: Reduce delay in prep script (for great reliability!)