Extend roadtypes / tramtypes examples and provide tests

[andythenorth]

Current

examples/roadtype_and_tramtype/ contains rudimentary examples for multiple roadtypes and a tramtype. These were copy-paste-edit of the railtype example, and contain railtype gui icons etc.

examples/road_vehicle includes one road vehicle compatible with some of the example roadtypes. It would be helpful to have more example road vehicles, covering the range of example roadtypes / tramtypes.

There are no tests for roadtypes / tramtypes.

Desired

• ensure the roadtype / tramtype examples are accurate, and provide fair coverage of the spec
• extend the example road vehicles and also provide a tram
• swap any relevant railtype sprites to roadtype / tramtype sprites
• provide test(s) for roadtypes and tramtypes

[andythenorth]

NRT docs are done in #46, examples next.