Organize files to avoid top-level clutter

[Rangi42]

We've previously reorganized https://github.com/gbdev/rgbds to have as few top-level files as possible, e.g. by creating cmake/ and docs/ subdirectories.

• Move the startup_time_test.gb test ROM and its startup_time_test.png screenshot to a testroms/ subdirectory
• Move all the .py files to a src/ directory