Remove '__init__.py' files from dbt.tests directory

[gshank]

Description

Because we combine files from multiple packages in the dbt.tests hierarchy, we can't have an 'init.py' file because it prevents doing that.

The problem we run into is that we can't install dbt-adapters using 'pip install -e .' into a local checkout of dbt-core. An error message occurs: ImportError: Error importing plugin "dbt.tests.fixtures.project": No module named 'dbt.tests.fixtures'

Acceptance criteria

It's possible to locally install dbt_adapters, dbt_common, and adapters without getting file import errors.

Impact to other teams

We need to make sure that the adapters work correctly.

Backports?

No. This is only in latest/versionless.