[CT-2919] Improved way of generating unit test yaml, via codegen or other techniques

Provide a simpler interface for generating the yaml for one unit test. While we like it as a clear way to represent the data structure of a unit test’s “recipe”, verbose yaml isn’t fun to write by hand — so we’re thinking of ways we can make this experience better, with some mix of [codegen](https://github.com/dbt-labs/dbt-codegen) and/or the dbt Cloud IDE.

The main work here is likely to not be code in dbt-core, so this ticket is for discussion and any changes that we need to make to support this.