setup: Stop swh.core.pytest_plugin from autoloading

Test Plan

tox

---

Migrated from D4181 (view on Phabricator)

conftest.py

@@ -3,6 +3,11 @@ import pytest
 
 from swh.core.cli import swh as _swhmain
 
+pytest_plugins = [
+    "swh.core.pytest_plugin",
+]
+
+
 # define tests profile. Full documentation is at:
 # https://hypothesis.readthedocs.io/en/latest/settings.html#settings-profiles
 settings.register_profile("fast", max_examples=5, deadline=5000)