Setup unit test for properly handling links

[brooksmtownsend]

After wasmCloud/provider-sdk-go#21 and realizing that there was a condition in some providers where handling links would fail, it would be great to setup a unit test that ensures

1. Links are handled and stored properly as source/target links when incoming on the link put topic
2. Links are handled and stored properly as source/target links when incoming on host data
3. A provider always starts with valid host data

Of course there's more to test with the SDK, but filling out some of these tests will give more confidence in future features.