[audio_core] Fractional position when mute

While adding resampler-related test cases and related refactoring,
in preparation for adding new resamplers, test cases exposed MTWN-296
in the mixer core:

To handle step_size limitations, the mixer uses modulo|numerator|
denominator values for any rate or position fractional components that
cannot be captured with the 13 bits of fixed-point fraction. For a
given mix pass, we calculate end position using these values, doing so
correctly in the usual case of producing every frame, but slightly
incorrectly in the optimization case of MUTED (whether explicitly
muted, or gain is simply too low to ever produce a value). This defect
would primarily be exposed when performing very fine-grained rate
adjustments.

This CL therefore contains the test-code refactoring so far, plus the
test cases that exposed the bug, and the product fix, as well as
additional test cases to verify that no other related bugs lurked.

Test: 15 new test cases, bringing the total to 170
('audio_mixer_tests --full' in basic_envs on all products.boards)

BUG: MTWN-296 #done

Change-Id: If41e20af77bcebbbad7aaf7d5301fded6c027dc3