Rectangles: sync expected test results and input data with problem-specifications.

[BethanyG]

Part of #1762

• Changed function name to rectangles in tests, example.py, and exercise stub to conform with canonical-data.json
• Changed function argument name to strings in example.py and exercise stub to conform with canonical-data.json
• Changed most single letter variable names to words in example.py for clarity.
• Changed comparisons to 0 to if not checks in example.py for clartiy.
• Replaced nested loop in search_corners function to a comprehension in example.py