Related to #1582

Description of the changes being introduced by the pull request:

When preparing the pr which was going to resolve #1582 I had to rename the test files
related to the old codebase. That way I was able to exclude the old test files when linting
the tests/ folder.
While I did that I realized I have to decide whether we want to lint and consequently rename
utils.py, test_utils.py, simple_server.py, and aggregate_tests.py after we remove the files testing the old code.
I decided that it's worth linting them for the following reasons:

• utils.py provides utilities that are used by our new code such as the TestServerProcess class
  which handles server handling, logging, and the run_sub_tests_with_dataset decorator which
  we used in multiple files.
• test_utils.py will be needed even after we remove the old test files as we want to have tests
  that verify that TestServerProcess from utils.py is working as expected
• simple_server.py is the only helper for server startup used in the new code base and
  more precisely to start the server in test_updater_ng.py
• aggregate_tests.py will be needed to aggregate the test files

That's why I decided to run the linters upon those files.
I tried to summarize the manual changes I had to make in the commit messages
as they weren't many.
Most of the changes are automatic and are result from black and isort.

Please verify and check that the pull request fulfills the following
requirements:

• The code follows the Code Style Guidelines
• Tests have been added for the bug fix or new feature
• Docs have been added for the bug fix or new feature