[GTK] check target looks broken

[GTK] check target looks broken
https://bugs.webkit.org/show_bug.cgi?id=184407

Reviewed by Carlos Garcia Campos.

It's pretty clear that nobody has been using check for a long time.

We got rid of check-for-webkitdom-api-breaks a long time ago, when we stopped autogenerating
the DOM API, but it's still used in the check target. Fix this.

Also, the check target for some reason runs itself from the Source subdirectory of the
builddir, but the test script expects to be run from the toplevel build directory. Fix this
too.

Add VERBATIM for good measure.

* Source/PlatformGTK.cmake:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@230937 268f45cc-cd09-0410-ab3c-d52691b4dbfc