audioconvert: fix resampler delay value and add test

Resampler delay for N taps is N/2-1 input samples.

Add test that checks this.

When input rate is varying, the resampler also accumulates additional
sub-sample delay. The resampler does not currently have API to report
the instantaneous sub-sample delay.  Add knownfail test for it.

spa/plugins/audioconvert/meson.build

@@ -185,6 +185,7 @@ test_apps = [
   'test-fmt-ops',
   'test-peaks',
   'test-resample',
+  'test-resample-delay',
   ]
 
 foreach a : test_apps