Merge #21254: test: Avoid connecting to real network when running tests

fa730e9157 test: Avoid connecting to real network when running tests (MarcoFalke)
fa1b713941 test: Assume node is running in subtests (MarcoFalke)

Pull request description:

  Introduced in #19884

ACKs for top commit:
  Sjors:
    ACK fa730e9157

Tree-SHA512: fe132a9ffe2fae1ab16857a3dec9839526fdf74d27a1ae794fbffca8356f639c4b916dc888b260281e9cc793916706c18d1687ebb5a076d4e1c481d218d308d3

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")