test: make BIP157 messages default-constructible (MESSAGEMAP compatibility)

In order to deserialize received or read messages via lookup in
MESSAGEMAP (e.g.: `t = MESSAGEMAP[msgtype]()`), the messages must have a
default constructor, i.e. there needs to be the possibility to
initialize them with zero arguments.

test/functional/test_framework/messages.py

@@ -1672,7 +1672,7 @@ class msg_getcfilters:
     __slots__ = ("filter_type", "start_height", "stop_hash")
     msgtype =  b"getcfilters"
 
-    def __init__(self, filter_type, start_height, stop_hash):
+    def __init__(self, filter_type=None, start_height=None, stop_hash=None):
         self.filter_type = filter_type
         self.start_height = start_height
         self.stop_hash = stop_hash
@@ -1722,7 +1722,7 @@ class msg_getcfheaders:
     __slots__ = ("filter_type", "start_height", "stop_hash")
     msgtype =  b"getcfheaders"
 
-    def __init__(self, filter_type, start_height, stop_hash):
+    def __init__(self, filter_type=None, start_height=None, stop_hash=None):
         self.filter_type = filter_type
         self.start_height = start_height
         self.stop_hash = stop_hash
@@ -1775,7 +1775,7 @@ class msg_getcfcheckpt:
     __slots__ = ("filter_type", "stop_hash")
     msgtype =  b"getcfcheckpt"
 
-    def __init__(self, filter_type, stop_hash):
+    def __init__(self, filter_type=None, stop_hash=None):
         self.filter_type = filter_type
         self.stop_hash = stop_hash