tests: stabilize the sorted output of list-tree.py on Windows

The test-largefiles-misc.t test was moving 'dir2\' before 'dir\' because while
'/' precedes most of the printable ASCII characters, '\' comes after numbers and
capital letters, among other symbols.

tests/list-tree.py

@@ -24,4 +24,4 b' def gather():'
         else:
             yield p
 
-print('\n'.join(sorted(gather())))
+print('\n'.join(sorted(gather(), key=lambda x: x.replace(os.path.sep, '/'))))