python/samba/tests: fix SamDB dummy replacement

In commit 6de9d878, a dummy SamDB lambda was added:

SamDB = lambda *x: None

The *x will only cover positional args. If we call it with kwargs:

samdb = SamDB(url=url)

We will get TypeError:

<lambda>() got an unexpected keyword argument 'url'

This commit fix this. It also fix PEP8 E731:

do not assign a lambda expression, use a def

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13542

Signed-off-by: Joe Guo joeg@catalyst.net.nz