Improve testing on Gemnasium Python

As per gitlab-org&3233 (comment 440562516)

• the case where DS_PIP_DEPENDENCY_PATH is not set isn't tested; see comment
• we need a test case for when PIP_CERT is explicitly provided; see comment
• and PIP_REQUIREMENTS_FILE isn't checked either
• the test project has a requirements.txt, but the cases where it's named requires.txt or requirements.pip are not tested