tests: harden audioio runner + clean up manual test docs

- run_serial_tests.py: pre-flight detects port held by other process
  (e.g. VS Code Serial Monitor) and reports the holder up front rather
  than spinning through opaque retries; add port-reappear wait,
  wider retry net (Errno 6/16, SerialException, "device in use"),
  and inter-test settle so a CDC drop in one test no longer cascades
  through the rest of the suite.
- README: drop hardcoded toolchain paths, fix contradictory stereo-WAV
  description, correct pan-sweep description (continuous equal-power
  crossfade, not stepped amplitude), align Test 3 sample-rate notes.
- Delete TESTING.md (was a near-duplicate of README.md).
- single_buffer_loop.py: use the same sample_rate for all four format
  variants so the test isolates format conversion, not playback rate.
- stereo_playback.py: use array initialiser instead of bytes literal
  for the stereo interleave buffer.
- wavefile_pause_resume.py: 30s wall-clock guard prints TIMEOUT rather
  than hanging the runner.