[plug-in] Create a test framework to test interactions between API and plugin runtime.

[sunix]

ATM, testing a plug-in API is about running a whole Theia app. It is not ideal in a developer flow neither when developing an API, neither when testing a plugin.
The requirements and goals of the framework would be

• to be able to run it from a JS test framework
• be able to mock plugin-in API implementation
• test interactions/usages between plugin core and plugin runtime without having to launch a Theia App

An example of usage of this framework could be the command API:

• command API is used through a plug-in in the plug-in runtime
• calls are then delegated to the plug-ext in Theia main node app and could appear differently at for the command being called.
• the framework would help in running test with the minimal set of components to be launched: command implementation, command plug-in api and plug-in code that invokes the command.