Test parlai/core/script.py

[stephenroller]

Patch description
Add tests for the abstract class in script.py, and the supercommand.

Testing steps
New CI.

$ coverage report parlai/core/script.py
Name                    Stmts   Miss  Cover
-------------------------------------------
parlai/core/script.py     137      2    99%

[klshuster]

big fan of this, hopefully this will catch the super subtle differences that sometimes come up between e.g. parlai tm and python -m parlai.scripts.train_model - ideally we can continue adding tests to this for maximum robustness