Provides a Catch::toString() overload for the following SFML classes, that support at least operator== and operator!=, and can thus be used in Catch assertion expressions:
* String
* Time
* Vector2<T>
* Vector3<T>
* VideoMode
* Color
* Rect<T>
The intermediate header UnitTests.hpp is introduced to ensure string conversion visibility.
Do not include <catch.hpp> directly any longer.
