test: add unit tests for main and actuation receivers

Exhaustive test cases for both classes, with 100% coverage. Adds `mock` dependency. Tests for actuation receiver do not yet pass.

ref: N25B-168

src/robot_interface/endpoints/receiver_base.py

@@ -12,7 +12,7 @@ class ReceiverBase(SocketBase, object):
         """
         Handle a message with the receiver.
 
-        :param message: The message to handle.
+        :param message: The message to handle, must contain properties "endpoint" and "data".
         :type message: dict
 
         :return: A response message or None if this type of receiver doesn't publish.