Add `assertInstanceOf` tests for newly introduced interfaces

[JakeQZ]

Some new interface types have been added to replace an orred type list for some methods. Possibly only CSSListItem (#1212) is relevant here. (Positionable was also added, but this is not used as a type for any parameter, and only exists to complement the Position trait.)

During those changes, tests should have been added to confirm that the classes replaced in the parameter lists actually implement the replacement all-encompassing type.

#1231 has a similar change, and includes relevant tests.