selftest: Add a python blackbox test for some misc (widelink) DFS tests

selftest: Add a python blackbox test for some misc (widelink) DFS tests

On master attempting to chdir into a nested dfs link

e.g. cd dfslink (works)
     cd dfslink/another_dfslink (fails)

[1] Add a test for this scenario (nested chdir)
[2] Add test for enumerating a dfs link in root of dfs share
[3] Add a test to check case insensitive chdir into dfs link on widelink
  enabled share

Add knownfails for tests 1 and 3

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Jeremy Allison <jra@samba.org>
BUG: https://bugzilla.samba.org/show_bug.cgi?id=15435