trivial: debian: clarify why installed tests get installed in a generic directory (Closes: #872458)
trivial: debian: clarify why installed tests get installed in a generic directory (Closes: #872458)

hughsie/fwupd,fwupd/fwupd,hughsie/fwupd,hughsie/fwupd,vathpela/fwupd,superm1/fwupd,fwupd/fwupd,superm1/fwupd,hughsie/fwupd,vathpela/fwupd,superm1/fwupd,vathpela/fwupd,fwupd/fwupd,vathpela/fwupd,fwupd/fwupd