Generate non-conflicting frozen upstream test requirements

[mattsb42-aws]

pyca/cryptography runs downstream tests for this project and other to verify that their changes have not broken identified downstream dependencies.

Events today caused the current way that we are handling this integration to be a problem. aws/aws-dynamodb-encryption-python#85

We need to define and automate creation of a special frozen test requirements file for upstream tests to use when testing this repository in order to avoid this happening again.