Smoke tests for statistical function output

[asmeurer]

Right now no output is tested at all for mean(), std(), and var(). But it would be nice to have some very basic smoke tests to ensure that at least the basic functionality is working.

Not sure what this should look like. If hypothesis is too hard we could just skip hypothesis and write some specific value tests for some nice valued arrays. Although it should be possible to just limit to values within reasonable ranges (to avoid floating-point issues) and check just the first few digits.