separate time-consuming tests to univariate_stats

The part that checks the consistency between expected stats (mean, var,
skewness, kurtosis, etc) and empirical stats have been moved to
univariate_stats.jl

Instead of making hard assertions, it prints absolute/relative
deviation in a table format for each distribution. We can investigate
whether something goes wrong by looking at the numbers.

We can add hard assertions after we have better understanding of the
range of the deviations.