Apply linters on tests/utils.py

Apply all 4 linters on tests/utils.py, so we can lint it in future and
not rename and exclude it.
It's expected that most if not all code in tests/utils.py will be
useful even when we remove the tests file on the old code.
Keep in mind when reviewing that type annotations where already added
in e2deff3

Black and isort changes where automatically made.
The only manual changes are:
- pylint disables
- function docstrings
- initializations of attributes of "TestServerProcess" __init__()
- additional asserts of the attributes types

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>

Author: Martin Vrachev