Use the TESTING_BRANCH env var for constraints
The TESTING_BRANCH environment variable is provided by the
run_tests.sh script and is derived from the .gitreview file.
This ensures that once the master branch becomes a stable
branch, the constraints from the stable branch in the
integrated repository will automatically get used once the
.gitreview file is updated.
To ensure that the required environment variables are present
we export them appropriately in run_tests.sh and modify the
tox configuration to pass them into the tox test.