add unit tests of scipy-based qualitative alpha and beta diversity metrics

[gregcaporaso]

Improvement Description To confirm that feature tables are converted to presence/absence data before the metric is applied (sklearn handles this for a lot of methods, so we need tests to ensure that we know if that ever changes).

References See https://github.com/qiime2/q2-diversity/issues/189 for more context.

[cherman2]

I am going to close this because it doesn't seem to have traction. Please re-open if you disagree