Introduce test for interface fragment on object type

[Kingdutch]

If interface A is implemented by object B then a fragment F on A should be
spreadable on a field returning object B. This is described in the spec
under section
5.5.2.3.1.

This test shows that this fails if a field from the fragment is also selected
in the top level selection. This is a valid use-case when a component lower
in the tree requires the data and the component making the query does too.

[jfrolich]

Thanks! I think I made it too type-safe so this case raises an error. I'll look into how this can be addressed.