gvfs-test: Check for URI string presence on CLI unmount

Turned out we can't easily check for 'Mount(0)' in `gvfs-mount -l`
output since that string can be present from a system mount,
either fstab or native volume monitor. Think of a mounted CD-ROM
drive.

We can't disable volume monitors either since there are tests that
require it. The best solution would be to check for the particular
URI but then we need to ensure the strings match (i.e. escaping issues),
bringing more stress to properly formatted URIs.

This fixes false unmount error reports when there was something else
mounted as 'Mount(0)'.

https://bugzilla.gnome.org/show_bug.cgi?id=691336