[V3] Make pytest fixtures available as a plugin (#1858)

* Move all fixtures to pytest plugin folder

* Add core dunder all

* Update other dunder all's

* Black reformat

redbot/pytest/data_manager.py

@@ -0,0 +1,24 @@
+import pytest
+
+from redbot.core import data_manager
+
+__all__ = ["cleanup_datamanager", "data_mgr_config", "cog_instance"]
+
+
+@pytest.fixture(autouse=True)
+def cleanup_datamanager():
+    data_manager.basic_config = None
+    data_manager.jsonio = None
+
+
+@pytest.fixture()
+def data_mgr_config(tmpdir):
+    default = data_manager.basic_config_default.copy()
+    default["BASE_DIR"] = str(tmpdir)
+    return default
+
+
+@pytest.fixture()
+def cog_instance():
+    thing = type("CogTest", (object,), {})
+    return thing()