test: MiniWallet: support default from_node for creating txs

If no `from_node` parameter is passed explicitely to the
`create_self_transfer` method, the test node passed in the course
of creating the MiniWallet instance is used.  This seems to
be the main use-case in most of the current functional
tests, i.e. in many instances the calls can be shortened.

test/functional/p2p_blocksonly.py

@@ -102,7 +102,7 @@ class P2PBlocksOnly(BitcoinTestFramework):
 
     def check_p2p_tx_violation(self):
         self.log.info('Check that txs from P2P are rejected and result in disconnect')
-        spendtx = self.miniwallet.create_self_transfer(from_node=self.nodes[0])
+        spendtx = self.miniwallet.create_self_transfer()
 
         with self.nodes[0].assert_debug_log(['transaction sent in violation of protocol peer=0']):
             self.nodes[0].p2ps[0].send_message(msg_tx(spendtx['tx']))