Unit testing docs

[dbeatty10]

Contributions

• I have read the contribution docs, and understand what's expected of me.

Link to the page on docs.getdbt.com requiring updates

https://docs.getdbt.com/docs/build/unit-tests

What part(s) of the page would you like to see updated?

We got some feedback that suggested the following updates to our documentation:

• Clarify fixture requirements for all dependencies (to avoid null values in output)
• Emphasize satisfying all model logic conditions (i.e. join / where clauses to avoid nulls)
• Under-the-hood code example of how unit tests compile

Additional information

No response

[runleonarun]

Thanks Doug! Let us know when you want a review on that PR.