[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/cog_manager.py

@@ -0,0 +1,13 @@
+import pytest
+
+__all__ = ["cog_mgr", "default_dir"]
+
+
+@pytest.fixture()
+def cog_mgr(red):
+    return red.cog_mgr
+
+
+@pytest.fixture()
+def default_dir(red):
+    return red.main_dir