Generating new random data on every test run makes it impossible to debug a
test failure. While in general we do want to generate random data to
thoroughly exercise the generator logic, if every new run generates new data,
the only thing we can do about an observed failure is shrug and ignore it. Add
logic to save data on failure, with corresponding logic to notice and reload
from a previously-generated save file.
In case of a merge collision, this version SUPERCEDES my previous efforts with
this file. (My other changes may still be in a backed-up merge request.) It is
okay to resolve collisions in favor of this version.
