tests: test-runner needs wayland-client

weston-test-runner.h includes wayland-util.h, therefore it needs wayland-client. A partial dependency with just compile_args might have been enough as it does not seem to use functions from wayland-util.c, but safer this way and no harm.

Fixes: https://lists.freedesktop.org/archives/wayland-devel/2020-January/041149.html