testutils: Terminate usage string with a new line
testutils: Terminate usage string with a new line

If a test binary is executed with an argument then usage
information is printed out (that no arguments are accepted and
what environment variables affect execution). The string is
printed onto stderr but it is not terminated with a newline
character producing not so nice output.

Signed-off-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

crobinso/libvirt,zippy2/libvirt,zippy2/libvirt,jfehlig/libvirt,libvirt/libvirt,crobinso/libvirt,libvirt/libvirt,olafhering/libvirt,olafhering/libvirt,jfehlig/libvirt,olafhering/libvirt,zippy2/libvirt,crobinso/libvirt,libvirt/libvirt,olafhering/libvirt,zippy2/libvirt,jfehlig/libvirt,crobinso/libvirt,jfehlig/libvirt,libvirt/libvirt