Add a CPAL playback test.

[mdwn]

A test has been added to the CPAL playback function. It's difficult to test this in its entirety due to its dependence on actual audio hardware, but we can at least verify that the output callback does what we expect. Some light refactoring was done as part of this.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 12.50000% with 14 lines in your changes are missing coverage. Please review.

Project coverage is 53.72%. Comparing base (dbd23cf) to head (edb6778).

Files	Patch %	Lines
src/audio/cpal.rs	12.50%	14 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #19      +/-   ##
==========================================
+ Coverage   52.53%   53.72%   +1.19%     
==========================================
  Files          20       20              
  Lines        1047     1048       +1     
==========================================
+ Hits          550      563      +13     
+ Misses        497      485      -12     

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