fact model for sources without tests

[dave-connors-3]

We can exclude them for now, but it might be interesting to at least have a fact down the line that could list all the sources that have or don't have tests, even if we don't build a dbt test on top of that.

Originally posted by @b-per in https://github.com/dbt-labs/pro-serv-dag-auditing/pull/6#discussion_r802619033

[graciegoheen]

Isn't our recommendation typically to add tests at the staging level, since this is the "most optimal version of our raw data"? Could be interesting to create a fact that could list all the direct children of sources that don't have tests... or something similar?