Merge bitcoin/bitcoin#30552: test: fix constructor of msg_tx

ec5e294 test: fix constructor of msg_tx (Martin Zumsande)

Pull request description:

  In python, if the default value is a mutable object (here: a class) it is shared over all instances, so that one instance being changed would affect others to be changed as well.
  This was the source of #30543, and possibly various other intermittent bugs in the functional tests, see
  bitcoin/bitcoin#29621 (comment).

  Fixes #30543
  Fixes #29621
  Fixes #25128

ACKs for top commit:
  sipa:
    utACK ec5e294. I believe some linters even warn about doing this.
  maflcko:
    ACK ec5e294
  vasild:
    ACK ec5e294 ❤️
  theStack:
    ACK ec5e294

Tree-SHA512: a6204fb1a326de3f9aa965f345fd658f6a4dcf78731db25cc905ff6eb8d4eeb65d14cc316305eebd89387aec8748c57c3a4f4ca62408f8e5ee53f535b88b1411

Author: fanquake

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)