Agent should not try to load extensions it can't use.

[CharliePoole]

When we run out of process, the agent tries to find and load extensions for all possible extension points, including those that are actually in the top-level engine and can never be installed.

The reason for this is that most extensionpoints are actually defined in the API assembly, which is not divided between nunit.engine and nunit.engine.core

This issue doesn't offer an immediate solution but is a placeholder for some fix we will need to make to avoid examining all possible extension points and to cut short the evaluation of extensions to the extent possible.

Suggestions that have been made (without evealuation)

• Split the API assembly
• Don't define extension ponits in the API assembly
• Implement some kind of "filtering" of extensions
• Discover the driver extension in the engine and pass the info to the agent