Test each unique violation

[sobolevn]

We need to create integration tests for all violations.

How should it look like?

0. We create a folder tests/fixtures/integration
1. We create a folder called tests/test_violations/ where all other tests will be located
2. Inside tests/test_violations/ we create a single test_VIOLATION_TYPE/ subfolder for all violation types
3. Then we create individual tests file for each violation class in the corresponding types subfolders

How should it work?

0. We create a fixture file with .env contents
1. We start a new process with Popen to lint this fixture with dotenv-linter command
2. We check the outputs

Example: https://github.com/wemake-services/wemake-python-styleguide/blob/master/tests/test_plugins.py