path_utils_ut: allow single / as well
From http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap03.html#tag_03_266
Multiple successive slashes are considered to be the same as one slash.
When running the tests on a musl libc system
get_dirname(p, PATH_MAX, "//foo//")
actually results in a / and not //
Reviewed-by: Michal Židek <mzidek@redhat.com>