Added tests for ensuring statistical correctness of approx_avg()

This diff consists of some additional tests that ensure the statistical correctness of `approx_avg()` function. The tests generate a sequence of tuples with a `gaussian` and `uniform` distribution and make sure that the true answer lies within the `approximate answer +/- error bars` produced by a number of 10% samples, 99% of the time.

I have explicitly seeded the distribution and sample generators to make the output of the tests deterministic.