This is the first in a series of commits that introduce components
that allow filters to be tested without MaxScale or backends.
Obviously the environment is not suited for all cases, but allow e.g.
the behaviour of the firewall filter to be tested in isolation.
The environment consists of helper classes loading and manipulating
modules and filters and mock-classes that provide the needed
scaffolding around a filter.
Currently all components exist under the test directory of the firewall
filter, but once proven they can be moved to a more general purpose
location.
e434c7ec66