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/minimal_test.py

@@ -0,0 +1,9 @@
+#!/usr/bin/env python3
+"""Minimal test to check if module works"""
+import sys
+import mcrfpy
+
+print("Module loaded successfully")
+print(f"mcrfpy has window: {hasattr(mcrfpy, 'window')}")
+print(f"mcrfpy.Frame exists: {hasattr(mcrfpy, 'Frame')}")
+sys.exit(0)