[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/admin.py

@@ -0,0 +1,20 @@
+from unittest.mock import MagicMock
+
+import pytest
+
+from redbot.cogs.admin import Admin
+from redbot.cogs.admin.announcer import Announcer
+
+__all__ = ["admin", "announcer"]
+
+
+@pytest.fixture()
+def admin(config):
+    return Admin(config)
+
+
+@pytest.fixture()
+def announcer(admin):
+    a = Announcer(MagicMock(), "Some message", admin.conf)
+    yield a
+    a.cancel()