python/samba/tests: fix SamDB dummy replacement

python/samba/tests: fix SamDB dummy replacement

In commit 6de9d878b, 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>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>