Need check-random-within

Consider the following function:

Exercise 4 Design a function between that takes two Posns as input and returns a Posn that is almost their midpoint, but randomly moved a little bit. The distance of random movement must be proportional to the distance between the two inputs, so use (drift (* 0.2 (dist ... ...))) twice, once to move the x coordinate and once to move the y coordinate. Here dist is from Assignment 6. So use dist, drift, and midpoint. For testing, remember to use check-random.

This is taken from https://cs.indiana.edu/classes/c211/lab14.html

Testing this function requires check-random, but if the tested Posns don't form a pythagorean triple, then you end up with inexact numbers and check-random refuses to compare. Similarly, you can't use check-within because it doesn't do randomness determinization.

I plan to write a PR to fix this.

Start with the Exercise 4 requirements and the existing check-random and check-within behavior described in the issue. Determine the intended semantics for comparing randomly generated Posns with inexact coordinates; done means a documented check-random-within capability handles this case without rejecting valid inexact results.