test: fix constructor of msg_tx

In python, if the default value is a mutable object (here: a class)
its shared over all instances, so that one instance being changed
would affect others to be changed as well.
This was likely the source of various intermittent bugs in the
functional tests.

test/functional/test_framework/messages.py

@@ -1294,8 +1294,11 @@ class msg_tx:
     __slots__ = ("tx",)
     msgtype = b"tx"
 
-    def __init__(self, tx=CTransaction()):
-        self.tx = tx
+    def __init__(self, tx=None):
+        if tx is None:
+            self.tx = CTransaction()
+        else:
+            self.tx = tx
 
     def deserialize(self, f):
         self.tx.deserialize(f)