tests: add test for save encountering duplicates

Add a test for save encountering duplicates in the index, both
for a file and a directory, which was fixed in the previous patch.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>