[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/rpc.py

@@ -0,0 +1,51 @@
+import pytest
+from redbot.core.rpc import RPC, RPCMixin
+
+from unittest.mock import MagicMock
+
+__all__ = ["rpc", "rpcmixin", "cog", "existing_func", "existing_multi_func"]
+
+
+@pytest.fixture()
+def rpc():
+    return RPC()
+
+
+@pytest.fixture()
+def rpcmixin():
+    r = RPCMixin()
+    r.rpc = MagicMock(spec=RPC)
+    return r
+
+
+@pytest.fixture()
+def cog():
+    class Cog:
+        async def cofunc(*args, **kwargs):
+            pass
+
+        async def cofunc2(*args, **kwargs):
+            pass
+
+        async def cofunc3(*args, **kwargs):
+            pass
+
+        def func(*args, **kwargs):
+            pass
+
+    return Cog()
+
+
+@pytest.fixture()
+def existing_func(rpc, cog):
+    rpc.add_method(cog.cofunc)
+
+    return cog.cofunc
+
+
+@pytest.fixture()
+def existing_multi_func(rpc, cog):
+    funcs = [cog.cofunc, cog.cofunc2, cog.cofunc3]
+    rpc.add_multi_method(*funcs)
+
+    return funcs