Adds a helper for stubbing component slots

What does this MR do?

This helper generates a component with only the slots you pass to it. This is useful for writing tests to check your scoped slots are working without having to fully mount the component. e.g.

wrapper = shallowMount(MyComponent, {
  store,
  propsData: { … },
  stubs: {
    'my-component': createSlotStub('header-content', 'footer-content'),
  }
});

This was needed for a test in !18382 (merged)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers