Create unit tests for SemanticManifest supported by a manifest fixture and utility builders

[QMalcolm]

Housekeeping

• I am a maintainer of dbt-core

Short description

As part of our Robust Testing Initiative we want to create generalized fixtures for our unit tests to make writing unit tests easier. To that end we want to add or update unit some unit tests to utilize the new fixture. Currently the SemanticManifest class is not unit tested at all, and it only requires a manifest (with some SL nodes).

Acceptance criteria

• There is a generic manifest unit test fixture
• There are builders to add SemanticModels, Metrics, and SavedQueries to a manifest
• There are unit tests for the SemanticManifest
• Reflect on test coverage of SemanticManifest during review

Suggested Tests

The work is the tests

Impact to Other Teams

None

Will backports be required?

Narp

Context

[ChenyuLInx]

Found lots of resusable things
https://github.com/dbt-labs/dbt-core/blame/ebc22fa26c9cb26b6deef86cc0a7ceb1ee3fb642/tests/unit/test_graph_selector_methods.py#L75