conftest: Declare swh.core pytest_plugin

As the other pytest plugin are declared.

The pytest.ini modification is required because it autoloads (as in swh-model!137 (closed) for example).

I tried to stop swh.core from autoloading its fixture but it's apparently more complex than anticipated [1]. I don't want to plunge in there just yet so here goes.

• [1] swh-core!172 (closed)

Test Plan

tox

---

Migrated from D4182 (view on Phabricator)

conftest.py

@@ -5,7 +5,11 @@
 
 import pytest
 
-pytest_plugins = ["swh.scheduler.pytest_plugin", "swh.storage.pytest_plugin"]
+pytest_plugins = [
+    "swh.scheduler.pytest_plugin",
+    "swh.storage.pytest_plugin",
+    "swh.core.pytest_plugin",
+]
 
 
 @pytest.fixture(scope="session")