Organize test suite: add README, move loose tests to proper directories

- Add tests/README.md documenting test structure and usage
- Move issue_*_test.py files to tests/regression/ (9 files)
- Move loose test_*.py files to tests/unit/ (18 files)
- tests/ root now contains only pytest infrastructure

Addresses #166

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/test_iter_flush.py

@@ -1,16 +0,0 @@
-#!/usr/bin/env python3
-import sys
-import mcrfpy
-print("1 - Creating scene", flush=True)
-scene = mcrfpy.Scene("test")
-print("2 - Getting children", flush=True)
-ui = scene.children
-print("3 - Creating frame", flush=True)
-frame = mcrfpy.Frame(pos=(0,0), size=(50,50))
-print("4 - Appending frame", flush=True)
-ui.append(frame)
-print("5 - Starting iteration", flush=True)
-for item in ui:
-    print(f"Item: {item}", flush=True)
-print("6 - Iteration done", flush=True)
-sys.exit(0)