test: Avoid connecting to real network when running tests

Can be reviewed with --word-diff-regex=.

test/functional/test_framework/util.py

@@ -368,6 +368,7 @@ def write_config(config_path, *, n, chain, extra_config=""):
         f.write("keypool=1\n")
         f.write("discover=0\n")
         f.write("dnsseed=0\n")
+        f.write("fixedseeds=0\n")
         f.write("listenonion=0\n")
         f.write("printtoconsole=0\n")
         f.write("upnp=0\n")