gnutls: use libtool to install test binaries

gnutls: use libtool to install test binaries

If libtool builds a binary it's best to use libtool to install it, as
otherwise you might install a wrapper script or need to make assumptions
about where libtool has put the real binary (as this isn't always .libs)

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>