Protect against importing production settings during tests.

Currently testsettings does `from settings import *` which attempts `from production_settings import *`. If one happens to have `production_settings.py` in ones checkout, tests can failing confusingly.

One solution is to remove the `from production_settings import *` from `settings.py` and instead do `from go.settings import *` in `production_settings` and then use `production_settings` as the Django settings module everywhere.

An alternative is to put dummy `production_settings` and `go.production_settings` modules in sys.modules in `go.testsettings` (might be confusing if `go.testsettings` is imported outside of tests).