tests: Use MAX_SCRIPT_ELEMENT_SIZE from script.py

p2p_segwit.py and test_framework/script.py both define a constant for
MAX_SCRIPT_ELEMENT_SIZE (=520 bytes), which is redundant.  This change
uses the constant defined in the script.py module for p2p_segwit.py.

test/functional/p2p_segwit.py

@@ -46,6 +46,7 @@
     CScript,
     CScriptNum,
     CScriptOp,
+    MAX_SCRIPT_ELEMENT_SIZE,
     OP_0,
     OP_1,
     OP_16,
@@ -1137,8 +1138,6 @@ def test_extra_witness_data(self):
     def test_max_witness_push_length(self):
         """Test that witness stack can only allow up to 520 byte pushes."""
 
-        MAX_SCRIPT_ELEMENT_SIZE = 520
-
         block = self.build_next_block()
 
         witness_program = CScript([OP_DROP, OP_TRUE])