tests: add audioio manual test suite for STM32F405/F407

Adds tests/circuitpython-manual/audioio/ with:

- wavefile_playback.py    — plays three WAV files (8-bit unsigned,
                            16-bit signed at 8 kHz and 44.1 kHz)
- wavefile_pause_resume.py — exercises pause()/resume() during playback
- single_buffer_loop.py   — loops a 440 Hz RawSample in all four
                            sample formats (u8, s8, u16, s16)
- run_serial_tests.py     — automates Tests 1–4 via mpremote: copies
                            files to the board and checks serial output
                            against expected patterns; exits 0/1 for CI
- README.md               — full test procedure including hardware setup,
                            build instructions, expected output for each
                            test, oscilloscope tips, and known limitations

Tests 1–4 are fully automated (requires: pip install mpremote).
Test 5 (soft-reset cleanup) remains a guided manual step.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>