Add a force-delete function to t/test.sh

Add a force-delete function to t/test.sh

Replace "rm -rf" with force-delete because "rm -rf" will proceed even
if it doesn't succeed.  This masked a problem (reported on Fedora)
where the root directory and other system directories had "r-xr-xr-x"
(0555) permissions, causing bup restore to create directories that "rm
-rf" couldn't remove.  As a result, subsequent tests would fail.

Signed-off-by: Ryan Brown <ryansb@csh.rit.edu>
[rlb@defaultvalue.org: edited commit message.]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>