Mocking container generated factories

Hey, I need some help with mocking factories created by the DI Container

config.neon

pageImporter:
			parameters: [param]
			class: PageImporter(..., %param%)

Presenter:

$importer = $this->context->createPageImporter('param');

Say I'm now writing a test for the presenter and I want to mock the behaviour of the service created by createPageImporter

There is no possibility of dynamically overriding the behaviour of the createPageImporter function (something like __call)

I've thought about creating “config snippets”, tiny configuration file overriding the main config with my own implementation, e.g.

testing < development:
	factories:
		pageImporter:
			parameters: [param]
			class: Tests\PageImporterStub(..., %param%)

However, addConfig assumes every file exists on its own including referenced config sections.

Having a complete copy of my basic config.neon with changes to mock only the necessary things is not feasible, I can't update all these configuration files when I change something in the main one.

I hope I'm just missing something trivial, thanks!