[tests] Add generate method to TestNode

Adds a generate() method to the TestNode class in the test framework.
This method intercepts calls to generate, imports a dewterministic
private key to the node and then calls generatetoaddress to generate the
block to that address.

Note that repeated calls to importprivkey for the same private keys are
no-ops, so it's fine to call the generate() method many times.

test/functional/test_framework/test_node.py

@@ -197,6 +197,25 @@ def wait_for_rpc_connection(self):
             time.sleep(1.0 / poll_per_s)
         self._raise_assertion_error("Unable to connect to bitcoind")
 
+    def generate(self, nblocks, maxtries=1000000):
+        self.log.debug("TestNode.generate() dispatches `generate` call to `generatetoaddress`")
+        # Try to import the node's deterministic private key. This is a no-op if the private key
+        # has already been imported.
+        try:
+            self.rpc.importprivkey(privkey=self.get_deterministic_priv_key().key, label='coinbase', rescan=False)
+        except JSONRPCException as e:
+            # This may fail if:
+            # - wallet is disabled ('Method not found')
+            # - there are multiple wallets to import to ('Wallet file not specified')
+            # - wallet is locked ('Error: Please enter the wallet passphrase with walletpassphrase first')
+            # Just ignore those errors. We can make this tidier by importing the privkey during TestFramework.setup_nodes
+            # TODO: tidy up deterministic privkey import.
+            assert str(e).startswith('Method not found') or \
+                str(e).startswith('Wallet file not specified') or \
+                str(e).startswith('Error: Please enter the wallet passphrase with walletpassphrase first')
+
+        return self.generatetoaddress(nblocks=nblocks, address=self.get_deterministic_priv_key().address, maxtries=maxtries)
+
     def get_wallet_rpc(self, wallet_name):
         if self.use_cli:
             return self.cli("-rpcwallet={}".format(wallet_name))