Allow selection of the samba4 process model for 'make test'

Allow selection of the samba4 process model for 'make test'

To choose the process model, set the environment variable
SAMBA_PROCESS_MODEL to the desired model. This will allow us to enable
the standard process model for some machines in the build farm without
enabling it for all of them. I don't want to just enable it
universally as I am concerned with total memory using during some of
the tests.