[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_data_manager.py

@@ -3,28 +3,10 @@ from pathlib import Path
 
 import pytest
 
+from redbot.pytest.data_manager import *
 from redbot.core import data_manager
 
 
-@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()
-
-
 def test_no_basic(cog_instance):
     with pytest.raises(RuntimeError):
         data_manager.core_data_path()