1. Add a script to generate a sorted list of most costly tests. A tests'
cost is roughly `runtime% / number of edits`. A slow test that's only
been updated once is much less valuable than a slow test that has
been updated 20 times: the latter test is catching more changes in the
type system.
2. Check in the results of running this script. I want to make the
skipping behaviour deterministic and the same for everybody, even though
you may get slightly better performance by examining only *your* test
changes.
3. Add code to skip tests until it reaches a 5% chance of missing an
edit. Right now this provides a 38% speedup.
Still not done:
4. Make this value configurable.
5. Make the CI configuration specify a 0% chance of missing an edit.
