Skip test_handling_of_incorrect_existing_linux_xattrs() when under fakeroot.

Don't try to do a loopback mount when running under fakeroot ("mount:
only root can do that").

Signed-off-by: Robert S. Edmonds <edmonds@debian.org>
[rlb@defaultvalue.org: adjust commit message]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>