t/test.sh: fix test failure when $top_dir has a dot-name

t/test.sh: fix test failure when $top_dir has a dot-name

Fix the admittedly rare case where we're running tests from a tree
that has a dot-name at its base, which only 'bup ls -a' will print.
This problem was originally encountered on a system that keeps mounts
under '/.autofs/'.

Signed-off-by: Yung-Chin Oei <yungchin@yungchin.nl>
Signed-off-by: Rob Browning <rlb@defaultvalue.org>