Phase out test_ functions

[filipsch]

See https://github.com/datacamp/pythonwhat/pull/257#issuecomment-392929988 for a first discussion.

More specifically:

• Manually update the exercises that use very seldomly used functions so we can remove those entirely.
• Go through all Python courses, and for the ones that still use test_ functions, set EXPOSE_PYTHONWHAT_V1=1. (or bake it in a Docker image)
• In pythonwhat, only if this environment variable these functions will be available.
• If the variable is not set, you will get a helpful error message specifying that this function is no longer exposed, and SCT writers should use the check_ alternative.

This is going to be a lot of work, but it's required to ensure a bright future with not too much maintenance pains.

[filipsch]

Deployed. If PYTHONWHAT_V2_ONLY=1, you can not use the old test_ functions.