test: Prevent failures caused by wrong monitor dir

Test suite uses custom servicedir with subset of monitor service files, however, list of available monitors is read from the system locations. Thus gvfs tries to spawn volume monitors, which are not available in the custom servicedir. Consequently, the following warnings are printed among others from "gio mount":

invoking IsSupported() failed for remote volume monitor with dbus name...

The warnings causes test suite failures, becasue it expects another output. This problem appeared recently due to the "gio mount" change, which now always call g_volume_monitor_get.

Set GVFS_MONITOR_DIR to the same dir, where is the custom session.conf, which ensures that it uses .monitor files from that dir and not from the system locations.