test: make p2p_handshake robust against timeoffset warnings

The test requires that limited nodes are not peered with  when
the node's system time exceeds ~ 24h of the node's chaintip
timestamp, as per PeerManagerImpl::GetDesirableServiceFlags.

By patching this test to modify the timestamp of the chaintip as
opposed to mocking the node's system time, we make it resilient
to future commits where the node raises a warning if it detects
its system time is too much out of sync with its outbound peers.

See bitcoin#29623

test/functional/p2p_handshake.py

@@ -62,6 +62,11 @@ def test_desirable_service_flags(self, node, service_flag_tests, desirable_servi
                 assert (services & desirable_service_flags) == desirable_service_flags
                 self.add_outbound_connection(node, conn_type, services, wait_for_disconnect=False)
 
+    def generate_at_mocktime(self, time):
+        self.nodes[0].setmocktime(time)
+        self.generate(self.nodes[0], 1)
+        self.nodes[0].setmocktime(0)
+
     def run_test(self):
         node = self.nodes[0]
         self.log.info("Check that lacking desired service flags leads to disconnect (non-pruned peers)")
@@ -71,10 +76,10 @@ def run_test(self):
                                           DESIRABLE_SERVICE_FLAGS_FULL, expect_disconnect=False)
 
         self.log.info("Check that limited peers are only desired if the local chain is close to the tip (<24h)")
-        node.setmocktime(int(time.time()) + 25 * 3600)  # tip outside the 24h window, should fail
+        self.generate_at_mocktime(int(time.time()) - 25 * 3600)  # tip outside the 24h window, should fail
         self.test_desirable_service_flags(node, [NODE_NETWORK_LIMITED | NODE_WITNESS],
                                           DESIRABLE_SERVICE_FLAGS_FULL, expect_disconnect=True)
-        node.setmocktime(int(time.time()) + 23 * 3600)  # tip inside the 24h window, should succeed
+        self.generate_at_mocktime(int(time.time()) - 23 * 3600)  # tip inside the 24h window, should succeed
         self.test_desirable_service_flags(node, [NODE_NETWORK_LIMITED | NODE_WITNESS],
                                           DESIRABLE_SERVICE_FLAGS_PRUNED, expect_disconnect=False)