test: known broken test for list(db.config) in python-cffi bindings

As of notmuch 0.34.2 [1], the python-cffi bindings make available the
configuration from both a config file and the database when accessing
Database.config like a dictionary.  It is therefore confusing that the
iterator operations only work on the configuration information stored
in the database.

[1]: d7f95724132bf658fd151630185899737e2ed829