Make most random tests deterministic

Fixes #47.

Not totally convinced this is *the best* way to fix this, but it at least is *a* way.

This mocks the `random/random#get` function when in most of the random tests. Some of the "no-bias" tests still should use the regular `Math.random`, so those are left alone (except for increasing the number of iterations). Note that I could not mock `Math.random` directly, since `random/random` saves it when it is defined.

From my testing this makes the tests *very* unlikely to randomly fail.