test: avoid non-loopback network traffic from node_init_tests/init_test

The test calls:
`AppInitMain()` -> `StartMapPort()` -> `StartThreadMapPort()` ->
`ThreadMapPort()` -> `ProcessPCP()` -> `PCPRequestPortMap()` ->
`CreateSock()` and on the returned value from `CreateSock()` it calls
the `Connect()` method.

Thus, change `BasicTestingSetup::BasicTestingSetup()` to set `-natpmp`
to 0. This way `node_init_tests/init_test` or other tests will not do
network activity due to `ThreadMapPort()`.

Also add a comment about `natpmp=0` in
`test/functional/test_framework/util.py`.

Also set `-dnsseed=0` in `BasicTestingSetup::BasicTestingSetup()` to
avoid DNS queries.

src/test/util/setup_common.cpp

@@ -199,6 +199,10 @@ BasicTestingSetup::BasicTestingSetup(const ChainType chainType, TestOpts opts)
     m_args.ForceSetArg("-datadir", fs::PathToString(m_path_root));
     gArgs.ForceSetArg("-datadir", fs::PathToString(m_path_root));
 
+    // Avoid non-loopback network traffic during tests.
+    gArgs.ForceSetArg("-dnsseed", "0"); // DNS queries are usually forwarded to upstream DNS servers.
+    gArgs.ForceSetArg("-natpmp", "0"); // NATPMP sends packets to the router.
+
     SelectParams(chainType);
     InitLogging(*m_node.args);
     AppInitParameterInteraction(*m_node.args);

test/functional/test_framework/util.py

@@ -576,7 +576,7 @@ def write_config(config_path, *, n, chain, extra_config="", disable_autoconnect=
         # in tests.
         f.write("peertimeout=999999999\n")
         f.write("printtoconsole=0\n")
-        f.write("natpmp=0\n")
+        f.write("natpmp=0\n") # Avoid non-loopback network traffic during tests.
         f.write("shrinkdebugfile=0\n")
         # To improve SQLite wallet performance so that the tests don't timeout, use -unsafesqlitesync
         f.write("unsafesqlitesync=1\n")