tests: allow cross-compiling the tests

I am trying to cross-compile from macOS for FreeBSD and this is currently
failing since the tests attempt to build a native binary that links
against the wayland-client and wayland-server libraries for the FreeBSD
system. I believe we should be building them for the target system and
not the current host (especially since there is no way to build
wayland-client and wayland-server for macOS, but I do want to check that
the files build correctly for FreeBSD).

Signed-off-by: Alex Richardson <Alexander.Richardson@cl.cam.ac.uk>
Reviewed-by: Simon Ser <contact@emersion.fr>