tests: Mark tests used so they don’t get removed at link time

Without this attribute, these macros were making Weston’s tests fail to
build with LTO enabled.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94602


Signed-off-by: Markus Trippelsdorf <markus@trippelsdorf.de>
Signed-off-by: Emmanuel Gil Peyrot <emmanuel.peyrot@collabora.com>
Tested-by: Emmanuel Gil Peyrot <emmanuel.peyrot@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>