Add enough test coverage for the resampler

[lu-zero]

Currently the code is not properly tested.

[xnorpx]

@lu-zero what is needed to ship 2.0? (I can help add test etc dependent what's blocking)

[lu-zero]

We need good test to make sure multichannel resampling is working, the current code for the stand-alone reworked resampler is competely bogus in that regard, I planned to rewrite it, but that requires tests first :)

[xnorpx]

ok, will take a look, written integration test when consuming the crate so maybe I can port some of those down. depending on how much using 2 one-channel resampler is iching :)