Write tests for RefdesRememberer

[amstan]

Together with issue #4, we should write a few tests for making sure refdeses do stay consistent. Something like a copy of servo_micro, and a ton of patches on top of it (all parented to one servo_micro, not on a chain, more like a star). Apply each of those patches then see if refdeses are sticking.

git checkout c718fbc~1 examples/servo_micro.py from https://github.com/google/pcbdl/commit/ba391e8febb21d93e5f2ecf990c85e3e0a6f1354 is a good idea.