[GTK] [CMake] Build the WebKit2 GObject API tests

https://bugs.webkit.org/show_bug.cgi?id=125683

Reviewed by Daniel Bates.

.:

* Source/PlatformGTK.cmake: Load the WebKit2 tests file if necessary.
* Source/cmake/FindATSPI.cmake: Added.
* Source/cmake/OptionsGTK.cmake: Look for AT-SPI, but don't fail if it isn't found.

Source/WebKit2:

* UIProcess/API/gtk/tests/CMakeLists.txt: Added.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@161016 268f45cc-cd09-0410-ab3c-d52691b4dbfc