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/unit/test_iter_only.py

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