webkit-gtk test: add missing load dependency
webkit-gtk test: add omission method
webkit-gtk test: require WebkitGtk::Omissons module

webkit-gtk does not provide in Homebrew upstream.
webkit-gtk test: add trivial WebKitGtk::WebView test
travis: add webkit-gtk installation packages
webkit-gtk2 test: add omissions module
webkit-gtk2 test: require WebkitGtk2::Omissions module
webkit-gtk2 test: add trivial WebKitGtk2::WebView test