[Feature] Enable dict format for ephemeral model unit tests

[fbertsch]

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

Allow ephemeral models to define unit tests using dict format. The rows for the unit tests could be written as YAML, easing both writability and readability.

Describe alternatives you've considered

Follow-up from #9686.

The sql format for unit tests allow us to define unit tests for ephemeral models, but are far less convenient than dict format:

• Unable to use anchors and references, which are especially useful for fields repeated in many/all unit tests
• Must supply data for all rows
• Linting/formatting is difficult

Who will this benefit?

Everyone who unit tests ephemeral models. This might make ephemeral models more useful.

Are you interested in contributing this feature?

If someone gives a rough pointer of where to contribute, I can do it

Anything else?

No response