Write tests for the module and its settings

@grevil could you list the tests @lrwebks should implement basically? (We shouldn't invest too much time but be sure it works)

I think we only need 4 pretty similiar tests:
- Disable vue and petite vue libraries, and check if both library definitions do not exist.
- Disable vue, enable petite vue and check if a library defintion for petite vue exists but not for vue.
- Disable petite vue, enable vue and check if a library defintion for vue exists but not for petite vue.
- Enable both libraries and check if both library defintions exist.

Note, to always clear the library defintion cache through libraryDiscovery->clearCachedDefinitions(); after enabling / disabling.