'If the mountpoint is unusable, for example because it is a regular file
instead of a directory, ``MountBlockDevice.run`` returns a ``Deferred``
that fires with a ``Failure`` given the reason.'

def test_mountpoint_error(self):

    mountroot = mountroot_for_test(self)
    intermediate = mountroot.child('mount-error-test')
    intermediate.setContent('collision')
    mountpoint = intermediate.child('mount-test')
    (_, mount_result) = self._run_test(mountpoint)
    failure = self.failureResultOf(mount_result, OSError)
    self.assertEqual(ENOTDIR, failure.value.errno)
