Unit testing docs

### Contributions

- [X] 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_