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>

tests/test_tools.py

@@ -24,7 +24,9 @@ import pytest
 def test_sbin_added_to_path():
     from diffoscope.tools import tool_required
 
-    @tool_required(os.listdir('/sbin')[0])
+    _, _, filenames = list(os.walk('/sbin'))[0]
+
+    @tool_required(filenames[0])
     def fn():
         pass