Extend fdktest with test helpers for executing handlers with a runner

[jsteenb2]

its important to include the runners as part of testing. A big component of this is the fact that we do have opinions in the runners, like File normalization, and will likely be extended in the future. We need a simple way to test handlers with the runner. The most obvious approach would be to have either an httptest instance, or do it all in process (nothing hits the wire), similar to issuing httptest.NewRequest|Records. With that test runner in place, we'd be able to take advantage of this within tests.

We could extend this further to provide a fluent API similar to how testttp works