test: Use int from_bytes and to_bytes over struct packing

This is done in prepration for the scripted diff, which can not deal
with the 0 literal int.

Author: MarcoFalke

test/functional/test_framework/messages.py

@@ -399,12 +399,12 @@ def __init__(self):
         self.vHave = []
 
     def deserialize(self, f):
-        struct.unpack("<i", f.read(4))[0]  # Ignore version field.
+        int.from_bytes(f.read(4), "little", signed=True)  # Ignore version field.
         self.vHave = deser_uint256_vector(f)
 
     def serialize(self):
         r = b""
-        r += struct.pack("<i", 0)  # Bitcoin Core ignores version field. Set it to 0.
+        r += (0).to_bytes(4, "little", signed=True)  # Bitcoin Core ignores the version field. Set it to 0.
         r += ser_uint256_vector(self.vHave)
         return r