[test-runner-*] mocking/stubbing es modules

[LarsDenBakker]

We should add patterns for mocking/stubbing es modules in tests. This can probably be a server plugin which works per test file.

It would be nice to base the format on import maps, in some possible future this import map could even just be injected into the page.

[daKmoR]

is now possible via https://github.com/modernweb-dev/web/tree/master/packages/dev-server-import-maps