tests/test_tools: fix test if /sbin contains a directory

The test_sbin_added_to_path test would then fail with an unhelpful
message (diffoscope.exc.RequiredToolNotFound: name_of_the_subdir) if
/sbin contains a subdirectory and os.listdir() returned it as the first
member.

Fixes: MR: !2
Thanks: Chris Lamb <lamby@debian.org> for the patch
Signed-off-by: Mattia Rizzolo <mattia@debian.org>