* Add an integration test to plugin template example

Dart unit tests don't exercise host-side plugin code at all, so the
example tests in the plugin template currently have very little
meaningful coverage. This adds an integration test to the example app
when creating a plugin, so that there's an example of how to actually
test that a complete round-trip plugin call works.

This is done as a separate template that's currently only used by the
plugin template because I don't know what a good example for a
non-plugin case would be that isn't largely just a duplicate of the
widget tests. However, the integration test pre-includes conditionals
around the parts that are plugin-specific so that it can more easily be
expanded to other use cases later (e.g., in
https://github.com/flutter/flutter/issues/68818).

Part of https://github.com/flutter/flutter/issues/82458

* Add integration test to expected dependencies of a plugin app

* Test fixes

* Make an explicit test case