Simplify test invocation command

[jacobtylerwalls]

We could add sensible defaults to ArchesTestRunner in order to shorten the invocation from

python manage.py test tests --settings=tests.test_settings

to just

python manage.py test

or even

./manage.py test

...if we make manage.py executable, which django-admin startproject does for you, but the arches command does not.

[jacobtylerwalls]

We already did this for the --pattern option.