[Robust Testing] Reorganize tests to match source code

[QMalcolm]

Housekeeping

• I am a maintainer of dbt-core

Short description

THIS IS A MEGA ISSUE INTENDED TO BE WORKED ON BY MULTIPLE PEOPLE DURING A TIME BOUND

Our unit tests are currently in files that don’t match the modules they are testing and are in a flat directory structure. This isn’t great because whenever we need to make a unit test, it’s hard to know if there is already a file the tests should go in and whether there are any similar tests to use as a model for the new tests.

Luckily there is a solution! Unit tests should be testing a specific function or class. As such it makes sense to reorganize the tests to live on paths and files that mirror where they live in the source code. For instance the test test_primary_key_model_constraint in tests/unit/test_infer_primary_key.py is testing the function infer_primary_key which lives in core/dbt/contracts/graph/nodes.py. Thus for the test location to be easier to reason about it should live in tests/unit/contracts/graph/test_nodes.py

Acceptance criteria

• If a test has a SetUp/TearDown convert these to fixtures
  • example: Change from unittest to pytest for manifestLoader #9960
• Move the class/function being unit tested to the appropriate location
  • If one test file contains tests for multiple code files, split it up

Suggested Tests

None. It's all tests already.

Impact to Other Teams

N/A

Will backports be required?

No

Context

No response

[ChenyuLInx]

Done with #10191