eca-sample-conversion: improve unit tests
eca-sample-conversion: improve unit tests

Add more test cases to catch problems with invertability of
the sample conversions.

Some of the new tests are marked non-fatal.

jeremysalwen/Ecasound-LV2,jeremysalwen/Ecasound-LV2,jeremysalwen/Ecasound-LV2,jeremysalwen/Ecasound-LV2,jeremysalwen/Ecasound-LV2