[CT-3046] Enable use of state:modified for detecting changed unit tests

[gshank]

Description

We can select unit tests using state:modified for changes to existing models.

In addition we'd like to be able to select unit tests that have changed using state:modified.

Changes include:

• Definition of the unit test
• Changes to the inputs(ie: fixtures)
• Changes to the logic of the model the unit test is for (depending on indirect_selection config)

Acceptance Criteria

Change only the contents of a unit test definition, and do dbt unit-test --select state:modified and the changed unit test should run.

Change the SQL for a given model and do dbt unit-test --select state:modified, the unit test on the changed model should run (depending on indirect_selection config)

Impact to other teams

None

Will backports be required?

No

Context