Split up and simplify maxCacheLength tests

A few changes to make these tests less scary:
* Invert `navigationWasCached` var to `didHitServer`, which feels a
  little more intuitive.
* Split the back button and forward button cases into separate tests.
* Reduce the number of assertions per test.
* Replace some comments with assertion text.
* Reduce `maxCacheLength` in the back button and forward button tests
  from 2 to 1 so that each test requires fewer steps.