tests: Fix "remote:branch" test to use ostree_parse_refspec

tests: Fix "remote:branch" test to use ostree_parse_refspec

ostree_validate_rev() validates bare ref names and does not accept
the "remote:ref" refspec syntax — the colon is not part of the
OSTREE_REF_REGEXP regex. The "remote:branch" form is a refspec,
which is parsed by ostree_parse_refspec().

Replace the incorrect ostree_validate_rev() call with
ostree_parse_refspec() and verify the parsed remote and ref
components, preserving the original test intent.

Fixes: ac10a27d ("pull: Fix GLib assertion crash on invalid UTF-8 ref names")