refactor: Reorganize async tests into tests/asyncio/ directory

Moved all async-specific tests into tests/asyncio/ following CPython's
organization pattern. Added comprehensive README.md with testing guidelines,
fixture documentation, and best practices.

Structure:
- test_basic.py (was test_async.py) - Basic async functionality
- test_acmd.py (was test_async_acmd.py) - Pattern A tests (8 tests)
- test_tmux_cmd.py (was test_async_tmux_cmd.py) - Pattern B tests (9 tests)
- test_hybrid.py (was test_async_hybrid.py) - Both patterns tests (7 tests)
- test_docstring_examples.py - Docstring verification tests (11 tests)
- README.md - Comprehensive testing guide with examples
- __init__.py - Package marker

Benefits:
- Clear separation of async tests
- Follows CPython's organizational patterns
- Comprehensive documentation for contributors
- Easy to run all async tests: pytest tests/asyncio/

Verified with: pytest tests/asyncio/ -v
Result: 37 passed in 1.20s (36 tests + 1 README doctest)

Author: tony

tests/asyncio/__init__.py

@@ -0,0 +1,5 @@
+"""Async tests for libtmux.
+
+This package contains all async-specific tests for libtmux's async support,
+organized following CPython's test structure patterns.
+"""