Add unit tests to `registry` package

### What problem are you facing?

We have integration tests that work on real data [here](https://github.com/upbound/upjet/blob/main/pkg/registry/meta_test.go) but no unit test that will let us understand individual units and manage changes more easily.

### How could Terrajet help solve your problem?

We can add `interface`s for scrapers and add unit tests for each of them.