tests: add stereo test and cross-platform support to audioio suite

Add Test 5 (stereo_playback.py) for verifying dual-DAC output. Update
run_serial_tests.py with cross-platform CIRCUITPY volume detection
(macOS/Linux/Windows), direct filesystem copy via shutil, and Python
3.7+ compatibility via `from __future__ import annotations`.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>