Add tests covering two drivers with the same name in separate registries

[amoeba]

We may already have some test coverage here but I'm filing this so we make sure we're covered. What happens when this sequence happens?

• t0: Driver "foo" exists in the private registry
• t1: Driver "foo" exists in the private registry and in the public registry

For both t, we need to test the behavior when the user (a) has "foo" installed at t0 and (b) does not. Also define and test what happens to our labels (i.e., "[private]".