Run perf test as a unit test to increase coverage

This exposed a few issues:

  1. We need to restore sinon stubs after creating them. There's an API for creating a sinon sandbox, but it mocks out some things we don't want, e.g. timers. If we keep running into bugs with stubs living too long, we can revisit this.
  1. Even with a broken stub, half the RecordStore tests were passing! I rewrote them to have more explicit expectations. Doing calculations in tests was always frowned upon at Google, and I think this is why. Your tests should break when your code is broken.

This should add `ExaminePage.js` to our code coverage metrics.
Did you mean to leave this here? Is so, a more explicit message might be good, e.g. "ExaminePage-perf-test is running as a unit test."