Make focus tests session-exclusive

Otherwise, tests running in parallel will steal the focus that
we are trying to test.
Revert "Adwaita: comment out the header-bar separator"

This reverts commit b4e7ada6ac52b9930ba8d14a5f4d5d3a009cf163.
Only show title button separator when there is something to separate

https://bugzilla.gnome.org/show_bug.cgi?id=729636
Adwaita: GtkSwitch size reduction and tweaks

See https://bugzilla.gnome.org/show_bug.cgi?id=734485