[Card Games] Refactor Tests and Add Sum() and Len() to Exercise

Per notes in PR2590, this exercise needs to have its tests and documents refactored. In particular:

• Refactor tests to include additional test scenarios and variants (with subTest) as needed.
• Refactor exemplar.py to omit the use of loops, since loops is not a prerequisite of this exercise, so cannot be used.
• Edit introduction.md and instructions.md to cover the use of built-ins len() and sum() (to be used in place of loops in the exemplar.py)

One suggestion to omit loops in exemplar.py is to return sum(hand)/len(hand) in card_average().

[J08K]

Yes! I actually am almost done, I got sidetracked. Did not realize it has already been 21 days… wow… Will submit a PR hopefully tomorrow.

[BethanyG]

@J08K – are you still working on this? Plz let me know. Thanks!