[GTK][CMake] Add a 'check' target

https://bugs.webkit.org/show_bug.cgi?id=126770

Reviewed by Daniel Bates.

* Source/PlatformGTK.cmake: Add a 'check' target that calls run-gtk-tests.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@161921 268f45cc-cd09-0410-ab3c-d52691b4dbfc