[Feature] Add unit tests to the Jinja `graph` object

[b-per]

Is this your first time submitting a feature request?

• I have read the expectations for open source contributors
• I have searched the existing issues, and I could not find an existing issue for this feature
• I am requesting a straightforward extension of existing dbt functionality, rather than a Big Idea better suited to a discussion

Describe the feature

Unit tests can be found in manifest.json, but they are not in the graph object.

This means that we can't do checks on those in dbt-project-evaluator (related issue: dbt-labs/dbt-project-evaluator#415 (comment))

We should add unit tests to the graph.

Describe alternatives you've considered

N/A

Who will this benefit?

This would allow us to improve dbt-project-evaluator and add checks related to unit-tests (there is no check on those today)

Are you interested in contributing this feature?

No response

Anything else?

No response