Increase unittest code coverage

[nickbattam]

Test coverage is lowest on the OpiModel widgets:

• Label
• TextMonitor
• TextInput
• GroupingContainer
• MenuButton
• ToggleButton.add_*_action
• Widget.add_children, get_parent, set_bg_color

[suv27]

Hey @nickbattam would you still want to increase the unittest coverage for this repo or maybe for pymalcolm?