The cups-pdf vm test previously waited for the
activation of `cups.service` before testing anything.
This method fails since
47d9e7d3d7
as cups auto-stops if it is not used,
causing the test framework to complain
that `cups.service` will never start.
The commit at hand alters the test so it
simply waits for `multi-user.target`.
We could also switch to `cups.socket`,
but `multi-user.target` seems to be more robust
concerning future changes in the cups mechanisms.
