tests: Avoid generating lots of output in itest-payload-link
tests: Avoid generating lots of output in itest-payload-link

We noticed this in a recent PR.  While I'm here, also only do
the `find` once, add `-type l` for good measure, and use our
built in `libtest.sh` assertion functions.

Closes: #1494
Approved by: giuseppe

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree