Build sf::Transformable tests

Transformable.cpp was originally compiled when I first submitted
PR #1973. While that PR was in review, #2012 got merged which
changed how tests are compiled. These two PR were in conflict so
when I went to resolve conflicts on #1973, I accidentaly removed
the line which added Transformable.cpp to the build. Because not
compiling this file caused no build breaks, nobody noticed until
after #1973 got merged. My bad, everybody.

test/CMakeLists.txt

@@ -35,6 +35,7 @@ if(SFML_BUILD_GRAPHICS)
         Graphics/Color.cpp
         Graphics/Rect.cpp
         Graphics/Transform.cpp
+        Graphics/Transformable.cpp
         Graphics/Vertex.cpp
     )
     sfml_add_test(test-sfml-graphics "${GRAPHICS_SRC}" SFML::Graphics)