test: resolve `basename "$0" .sh` once for all in test-lib.sh

test-lib.sh sometimes did equivalent of `basename "$0" .sh`, sometimes
skipping the basename part and sometimes .sh part. This worked as
we never had path components in $0 (more than ./) nor .sh ending.

Now the equivalent of `basename "$0" .sh` is done once and used
everywhere. In the future we may have .sh suffix in test names
-- removing those is a good idea.