Test harness for plugin unit testing

I'm writing a custom plugin for our GraphQL server and would like to write some unit tests for it. I'm finding that I have two choices here: Figure out how to mock out the various objects that are passed into the plugin or spin up an ApolloServer with the plugin installed (and all the dependencies that entails).

It would be nice if there were a test harness that would make it easy to test my plugin in isolation, without having to integrate it into a full Server, somehow. Perhaps something that would make it easier to create the various objects passed to `requestDidStart`, `executionDidStart`, etc.

Start with the plugin lifecycle entry points named in the issue, including requestDidStart and executionDidStart, and compare the current mocking and full ApolloServer approaches. Done would be a defined way to exercise those hooks in isolation without starting a full server or supplying all its dependencies.

Written by the indexing model from the issue text.