audiomixertest: improve codebase to make it work correctly with wp 0.5

audiomixertest: improve codebase to make it work correctly with wp 0.5

- Replace g_warning() with fprintf(), so that we don't need to enable
  WIREPLUMBER_DEBUG to actually read those messages.
- Make it so that the help message can be printed before connecting
  to pipewire and waiting for controls.
- Remove the race condition mitigation hack that was previously there,
  in case ensure_controls() did not discover all controls initially.
- Lock the audiomixer properly when using its methods and structures.

Bug-AGL: SPEC-4934
Signed-off-by: George Kiagiadakis <george.kiagiadakis@collabora.com>
Change-Id: Ia4b9448dee83838858153f1c1c744e962294cd4e