[tests] Remove deprecated addwitnessaddress call from feature_nulldummy

addwitnessaddress is deprecated. Replace the call to addwitnessaddress
with a call to getnewaddress(address_type='p2sh-segwit')

Author: jnewbery

test/functional/feature_nulldummy.py

@@ -41,15 +41,15 @@ def set_test_params(self):
         self.setup_clean_chain = True
         # This script tests NULLDUMMY activation, which is part of the 'segwit' deployment, so we go through
         # normal segwit activation here (and don't use the default always-on behaviour).
-        self.extra_args = [['-whitelist=127.0.0.1', '-vbparams=segwit:0:999999999999', '-addresstype=legacy', "-deprecatedrpc=addwitnessaddress"]]
+        self.extra_args = [['-whitelist=127.0.0.1', '-vbparams=segwit:0:999999999999', '-addresstype=legacy']]
 
     def skip_test_if_missing_module(self):
         self.skip_if_no_wallet()
 
     def run_test(self):
         self.address = self.nodes[0].getnewaddress()
         self.ms_address = self.nodes[0].addmultisigaddress(1, [self.address])['address']
-        self.wit_address = self.nodes[0].addwitnessaddress(self.address)
+        self.wit_address = self.nodes[0].getnewaddress(address_type='p2sh-segwit')
         self.wit_ms_address = self.nodes[0].addmultisigaddress(1, [self.address], '', 'p2sh-segwit')['address']
 
         self.coinbase_blocks = self.nodes[0].generate(2)  # Block 2