bugfix: mutmut should pick up the config before running the original test suite

[atmay]

Bug: mutmut reads the config file stored in mutmut_config.py only after it runs the initial set of tests. Some tests (like end-to-end, integration tests etc) can't pass without certain infrastructure and are not even supposed to be part of mutation testing process.

As a result they fail before mutmut accesses the config file with runner command and mutmut can't proceed and crashes as well.

Fix: Reading configs prior to running initial set of tests allows to specify test suits you want to run and avoid crashes.

Example: Let's say, you have unit tests in tests/unit/ folder and you want to run only them. You can configure your mutmut_config.py like so:

from pathlib import Path

def init():
    test_files = (Path(__file__).parent / "tests/unit").rglob("test*.py")

def pre_mutation(context):
    context.config.test_command = "poetry run pytest tests/unit"

[boxed]

Thanks!