Added string conversions for equality-comparable classes

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.

test/src/UnitTests.hpp

@@ -0,0 +1,58 @@
+// Header for SFML unit tests.
+//
+// For a new test case, include this header and not <catch.hpp> directly.
+// This ensures that string conversions are visible and can be used by Catch for debug output.
+
+#ifndef SFML_UNITTESTS_HPP
+#define SFML_UNITTESTS_HPP
+
+#include <SFML/System/Vector2.hpp>
+#include <SFML/System/Vector3.hpp>
+#include <SFML/Graphics/Rect.hpp>
+#include <sstream>
+#include <catch.hpp>
+
+
+// Forward declarations for non-template types
+namespace sf
+{
+    class Color;
+    class String;
+    class Time;
+    class VideoMode;
+}
+
+// String conversions for Catch framework
+namespace Catch
+{
+    std::string toString(const sf::String& string);
+    std::string toString(sf::Time time);
+    std::string toString(const sf::VideoMode& videoMode);
+    std::string toString(const sf::Color& color);
+
+    template <typename T>
+    std::string toString(const sf::Vector2<T>& vector)
+    {
+        std::ostringstream stream;
+        stream << "(" << vector.x << ", " << vector.y << ")";
+        return stream.str();
+    }
+
+    template <typename T>
+    std::string toString(const sf::Vector3<T>& vector)
+    {
+        std::ostringstream stream;
+        stream << "(" << vector.x << ", " << vector.y << ", " << vector.z << ")";
+        return stream.str();
+    }
+
+    template <typename T>
+    std::string toString(const sf::Rect<T>& rect)
+    {
+        std::ostringstream stream;
+        stream << "(left=" << rect.left << ", top=" << rect.top << ", width=" << rect.width << ", height=" << rect.height << ")";
+        return stream.str();
+    }
+}
+
+#endif // SFML_UNITTESTS_HPP