[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/economy.py

@@ -0,0 +1,15 @@
+import pytest
+
+__all__ = ["bank"]
+
+
+@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