Add better support for incremental in standard source tests

[cgardens]

Tell us about the problem you're trying to solve

• The standard source tests make some goofy assumptions to figure how to mutate a given catalog to be used for testing incremental. The assumptions are super brittle. They were good for getting the first 2 integrations out the door, but it's time to make it sturdier.

Describe the solution you’d like

We need to add support for incremental into the interface so there is less implicit stuff going on.

┆Issue is synchronized with this Asana task by Unito

[cgardens]

@sherifnada and I discussed offline. Agreed that there's no work that needs to be done now. In the future we'd like to test for individual records in the standard tests. (or do it in specific integration tests). Either way doesn't need to be done now.