* Add Windows unit tests to plugin template

Adds an example native unit test to the plugin template for Windows,
matching the format we use for our 1P plugin example app unit tests.
Once these have been added for all platforms+languages, they will be
documented on a new plugin development page to explain their use.

Since we don't appear to be running our current plugin e2e tests for
Windows, this adds a new configuration to run them. I haven't
`led`-tested this, so it may not work, but this will give a starting
point for getting them running.

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

* Minor fix

* Add test owner

* Fix typo

* Fix test feature flag