Reduce the amount of snapshot tests

[Caleb-Ellis]

While updating the components to use Vanilla 1.8.0 styling/markup (from 1.6.6) I found that pretty much every component that changed had their tests fail. This was due to the tests being very tightly coupled with the markup (almost 1:1 coupling in fact) meaning that the tests are extremely brittle. I think we should reduce the amount of snapshot tests considerably, in favour of simpler unit tests.

As an example, currently one of the MediaObject tests checks if the large prop works by snapshotting the markup. It would be much simpler to just check if the relevant class (p-media-object--large) is present, rather than the whole markup structure match the snapshot.