[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/cogs/test_economy.py

@@ -1,16 +1,5 @@
 import pytest
-
-
-@pytest.fixture()
-def bank(config, monkeypatch):
-    from redbot.core import Config
-
-    with monkeypatch.context() as m:
-        m.setattr(Config, "get_conf", lambda *args, **kwargs: config)
-        from redbot.core import bank
-
-        bank._register_defaults()
-        return bank
+from redbot.pytest.economy import *
 
 
 @pytest.mark.asyncio