tests/g-file-info-filesystem-readonly: unmount lazily
tests/g-file-info-filesystem-readonly: unmount lazily

fusermount -z behaves like umount --lazy, which is documented thus:

> Detach the filesystem from the file hierarchy now, and clean up all
> references to this filesystem as soon as it is not busy anymore.

Without this, the call to `fusermount -u` often fails with:

  /usr/bin/fusermount: failed to unmount /home/wjt/src/gnome/glib/_build/dir_bindfs_mountpoint: Device or resource busy

which causes the subsequent call to bindfs to fail:

  fuse: mountpoint is not empty
  fuse: if you are sure this is safe, use the 'nonempty' mount option

It's not clear what is causing the mount to be busy. Inserting a
g_usleep (100 * 1000) before the calls to `fusermount -u` also works to
make the problem go away, but for the purposes of this test the
important point is that the mount is detached from the directory, for
which a lazy unmount is fine.

Fixes #1590.

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib