Create DiGraph fixtured and unit test Graph Class

[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 Graph class is not unit tested at all. We could unit test a majority of the the class by having a DiGraph fixture. Additionally there are other parts of the code base, such as the GraphQueue which also need a DiGraph fixture. Thus creating the fixture would make it easier to create unit tests for other parts of the codebase.

Acceptance criteria

• There is a generic DiGraph unit test fixture
• There are unit tests for the config Graph class

Suggested Tests

The work is the tests

Impact to Other Teams

None

Will backports be required?

Narp

Context

[QMalcolm]

I believe a graph fixture could be created by taking the manifest fixture created from a manifest fixture (perhaps the one created during #9665), and running it through a linker as is done in the compile function

[ChenyuLInx]

@QMalcolm sync up with @peterallenwebb about how this ties to removing networkx(potentially).