tests: Remove an unecessary Makefile.am variable
tests: Remove an unecessary Makefile.am variable

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