view handler test cleanups

These are some cleanups to the view handler tests.  There are two
main goals here:

* reduce coupling between tests by removing the `FlagViewHandler`
  and instead using a specific `SinatraRunnerViewHandler` and a
  local `TestViewHandler` (old test view handler renamed to
  `EmptyViewHandler`.
* rework the main view handler unit tests to our more modern conventions
  and flow.  No major changes here - tests just organized better and
  more consistently.

This is prep for some coming view handler changes related to native
template rendering.