refactor(tests): Factor common patterns in tempo testsHEADmain

Reduced test code duplication by adding helpers within each file:
- setup_audio_test(): eliminates 6-line init boilerplate
- simulate_tempo(): replaces repeated tempo simulation loops
- simulate_tempo_fn(): supports variable tempo with lambda

Results:
- test_variable_tempo.cc: 394→296 lines (-25%)
- test_tracker_timing.cc: 322→309 lines (-4%)
- Total: -111 lines, all 31 tests passing

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

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