[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

tests/core/test_cog_manager.py

@@ -2,19 +2,10 @@ from pathlib import Path
 
 import pytest
 
+from redbot.pytest.cog_manager import *
 from redbot.core import cog_manager
 
 
-@pytest.fixture()
-def cog_mgr(red):
-    return red.cog_mgr
-
-
-@pytest.fixture()
-def default_dir(red):
-    return red.main_dir
-
-
 @pytest.mark.skip
 @pytest.mark.asyncio
 async def test_ensure_cogs_in_paths(cog_mgr, default_dir):