Make the transform streams actually propagate backpressure.

The previous approach was too naive. This one correctly waits to do a transform until backpressure has eased up.
Initial draft.

This crashes if I set underlyingSourceRate to 2!
Add a transform in the middle, and some comments
Fancy, exhaustive, and fairly useless test runner.

The output is not something you can make decisions on. We need aggregate data and a way of comparing between runs, and something you can watch as it runs. And it's probably still too slow.
Make the output more useful.

Now it scrolls by one line per test.
Add backpressure and record number of times it is triggered
A stab at a quicker set of possibilities
Candidate set of ~400.
setting underlyingSourceRate = 'sync' blows things up after one test. not sure why.
Fixed that. Now it's somewhat working. Still probably takes too long...