Introduce stubComponent helper

@vue/test-utils 1.x will change behavior for stubs. Now they will
render all available slots instead of defaults. To simplify
migration and taking care of emerging pattern of "stubbing with
props" we're introducing `stubComponent` helper, which will copy
props and model of stubbed component and also ensure that stubbed
component still could be found with default vue-test-utils finders
(this removes several ugly workarounds in our codebase)