test-lib.sh: do not barf under --debug at the end of the test
The original did "does $remove_trash exist? Then go one level above
and remove it". There was no problem under "--debug", where
the variable is left empty, as the first "test -d $remove_trash" would
have said "No, it doesn't".
With the check implemented in the previous step, we'd always get an
error under "--debug".
Noticed-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>