Tests for source overriding

[beckjake]

Describe the feature

Sources in the root package can override the sources defined in dependencies. That's good! But we don't test that this is the case, and could easily regress. That is bad, especially because we want to document this behavior and tell users they can rely on it.

Describe alternatives you've considered

No tests, probably break this in a future release, upset people.

Additional context

N/A

Who will this benefit?

Tests benefit everyone.