Add support for unit-testing async code in the editor

[dubois]

This overlaps #18 and addresses #9. This approach differs in that it uses a Unity-provided package for scheduling coroutines at edit time. It also includes worked examples of how to run async unit tests.

It requires that end users install the package com.unity.editorcoroutine. If the end user is using Unity 2018 or below, they must also manually define HAVE_EDITOR_COROUTINES. In Unity 2019 and above, the define is handled automatically by the included .asmdef file.