gvfs-test: Robustify MTP test

Don't assume a particular odering of listing the top-level directory.

For unmount success, verify that our Mount object is gone, instead of our URI
not appearing anywhere in the "gvfs-mount -li" output; the latter catches the
Volume as well, which we don't expect to go away.
gvfs-test: Refresh MTP record

- DCIM/100CANON/IMG_000?.JPG are from test/files/source-gphoto/

- sine.ogg was generated with
  gst-launch-1.0 audiotestsrc num-buffers=10 ! audioconvert ! \
  audioresample ! vorbisenc ! oggmux ! filesink location=sine.ogg

- Record started with
  umockdev-record -i /dev/bus/usb/001/017=test/files/mtp_xperia.ioctl /usr/lib/gvfs/gvfsd

- Recorded commands (in a different terminal):
  gvfs-mount 'mtp://[usb:001,017]'
  gvfs-ls 'mtp://[usb:001,017]/SD-Karte'
  gvfs-ls 'mtp://[usb:001,017]/SD-Karte/Music'
  gvfs-ls 'mtp://[usb:001,017]/SD-Karte/Music/GStreamer - The Test Sine'
  gvfs-info 'mtp://[usb:001,017]/SD-Karte/Music/GStreamer - The Test Sine'
  gvfs-info 'mtp://[usb:001,017]/SD-Karte/Music/GStreamer - The Test Sine/sine.ogg'
  gvfs-cat 'mtp://[usb:001,017]/SD-Karte/Music/GStreamer - The Test Sine/sine.ogg'
  gvfs-cat 'mtp://[usb:001,017]/SD-Karte/hello.txt'
  gvfs-info 'mtp://[usb:001,017]/SD-Karte/hello.txt'
  gvfs-info 'mtp://[usb:001,017]/SD-Karte/DCIM/100CANON/IMG_0001.JPG'
  gvfs-cat 'mtp://[usb:001,017]/SD-Karte/DCIM/100CANON/IMG_0001.JPG'
  gvfs-mount -u 'mtp://[usb:001,017]'