Summary:
This sets up the initial infra that is to be used by plugin devs to test plugins.
There is not much yet to see, as there is no state or message sending yet. But at least the life cycle of plugins can be test, things are strongly typed and everything is in the place where it should be :)
N.b. the import difference with these utils and the createFlipperMock utilities in Flipper are
1. this testing infra is entirely inside flipper-plugin package, so that plugin devs don't need flipper as a dependency
2. this testing infra doesn't provide abstractions for plugin / device / client switching; it tests plugins purely in isolation of the rest of the world (except for firing `onConnect` / `onDisconnect` which is normally the effect of switching plugins)
Reviewed By: nikoant
Differential Revision: D22255262
fbshipit-source-id: b94ccbab720d2b49428a646aed3c55af71a5bc80
