[PBCKP-358] sort directories to not skip 'base/1' because of 'base/12699'

Looks like os.walk may yield dirs unsorted.
This way "check if directory already here as part of larger directory"
man mistakenly skip short 'base/1' as "part of" 'base/12699'

tests/helpers/ptrack_helpers.py

@@ -1799,7 +1799,7 @@ class ProbackupTest(object):
                         )
 
         for root, dirs, files in os.walk(pgdata, topdown=False, followlinks=True):
-            for directory in dirs:
+            for directory in sorted(dirs):
                 directory_path = os.path.join(root, directory)
                 directory_relpath = os.path.relpath(directory_path, pgdata)