Split pip requirements files into securedrop and test requirements

This is the most logical split for the pip requirements. Every
environment needs the securedrop-requirements.txt, but only environments
that need to run the tests locally (development and staging) need
test-requirements.txt.

Tested by running `vagrant up development && vagrant up /staging$/`.
Merge pull request #675 from freedomofpress/consolidate-pip-requirements

Split pip requirements files into securedrop and test requirements