[autotests/integration] Add test case for global shortcuts with Fx

New test which tries to trigger Alt+F3 which does not work due to the
behavior how xkbcommon calculates consumed modifers. The combination
Ctrl+Alt+F3 generates a keysym (vt switching) so just pressing F3
already consumes ctrl and alt modifier.

For more information see:
 * https://github.com/xkbcommon/libxkbcommon/issues/17
 * https://bugs.freedesktop.org/show_bug.cgi?id=92818

CCBUG: 368989