[test framework] make it easier to fast-forward setmocktime

Have each TestNode keep track of the last timestamp it called
setmocktime with, and add a bumpmocktime() function to bump by a
number of seconds. Makes it easy to fast forward n seconds without
keeping track of what the last timestamp was.

test/functional/test_framework/test_node.py

@@ -144,6 +144,8 @@ class TestNode():
         self.p2ps = []
         self.timeout_factor = timeout_factor
 
+        self.mocktime = None
+
     AddressKeyPair = collections.namedtuple('AddressKeyPair', ['address', 'key'])
     PRIV_KEYS = [
             # address , privkey
@@ -324,6 +326,15 @@ class TestNode():
         assert not invalid_call
         return self.__getattr__('generatetodescriptor')(*args, **kwargs)
 
+    def setmocktime(self, timestamp):
+        """Wrapper for setmocktime RPC, sets self.mocktime"""
+        if timestamp == 0:
+            # setmocktime(0) resets to system time.
+            self.mocktime = None
+        else:
+            self.mocktime = timestamp
+        return self.__getattr__('setmocktime')(timestamp)
+
     def get_wallet_rpc(self, wallet_name):
         if self.use_cli:
             return RPCOverloadWrapper(self.cli("-rpcwallet={}".format(wallet_name)), True, self.descriptors)
@@ -699,6 +710,13 @@ class TestNode():
 
         wait_until_helper(lambda: self.num_test_p2p_connections() == 0, timeout_factor=self.timeout_factor)
 
+    def bumpmocktime(self, seconds):
+        """Fast forward using setmocktime to self.mocktime + seconds. Requires setmocktime to have
+        been called at some point in the past."""
+        assert self.mocktime
+        self.mocktime += seconds
+        self.setmocktime(self.mocktime)
+
 
 class TestNodeCLIAttr:
     def __init__(self, cli, command):