Execution test cases

Define stdin and expected stdout for each suite. Saving writes files next to server.py (under test_suites/<set>/case_01.stdin and case_01.expected, numbered in order).

Local server only. This page only talks to your companion API. gcc, running the compiled binary with those stdin values, and writing suite files all happen on the computer where you run python3 server.py — not on GitHub Pages and not inside the browser. Use the same server URL as on the Grade page (often http://localhost:5001).