tests: Handle modifier event in test-client.c
tests: Handle modifier event in test-client.c

We need the handle the newly added modifiers event.

https://bugs.freedesktop.org/show_bug.cgi?id=51567

giucam/weston,Tarnyko/weston-xdg_surface_present,xorgy/weston,krezovic/weston,sir-murray/weston,jonnylamb/weston,krezovic/weston,kwm81/weston,Fantu/compositor-spice,udoprog/weston,Gnurou/weston,udoprog/weston,giucam/weston,mchalupa/weston,mchalupa/weston,eyolfson/weston,sir-murray/weston,Fantu/compositor-spice,Fantu/compositor-spice,xorgy/weston,Gnurou/weston,jonnylamb/weston,eyolfson/weston,Tarnyko/weston-xdg_surface_present,kwm81/weston