Tests: cts-cli: fix thinko leading to system-wide shadow files

In particular, building RPM package and triggering %check lead
to cts-cli trying to work with shadow copies located under
/var/lib/pacemaker/cib -- and failing because that path doesn't
exist at that time (unless the buildroot is already spoiled).
This change was unintentionally brought with 1d0d0eff2 commit.

Note that "mktemp" is not reliably portable (not POSIX'd),
but there were other occurrences already.