Refactor AudioProcessingTest to match current audio processing code

The unittest suite AudioProcessingTest was written long ago at a time
when the audio processing code was very different from what it is now
and has only received minor updates during 10+ years.

It has since long reached a state where its core testing has become
irrelevant. E.g., its assumptions that sample-wise differences
should be low in the output for cases where downmixing and internal
resampling is used are no longer accurate enough for the test to pass.

To get the test to pass, a common workaround has over the years been
to reduce the accuracy in the comparison of the output against the
desired reference (i.e., reduce the tabularized threshold
expected_snr).
The reason for that workaround is that there is essentially no way to
tell what magnitude of difference is correct or incorrect.
That has resulted in that in several cases, that threshold has been
set to 0, effectively deactivating the test checks.

This CL refactors the test to remove the code that evaluates the
accuracy of the output (which is no longer relevant) and keep its
iteration over different channels counts and sample rates.
This reduces the test from doing soft-bitexactness to be a crash-test
that makes sure that the code is verified to run for relevant
configurations.


Bug: webrtc:42230301
Change-Id: I52a7753d61db627cbf6f10dbc1a1d2bfe87deddf
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/446920
Commit-Queue: Per Åhgren <peah@webrtc.org>
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Reviewed-by: Jesus de Vicente Pena <devicentepena@webrtc.org>
Reviewed-by: Henrik Lundin <henrik.lundin@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46841}