test: adds seednode functional tests

Adds functional tests to test the interaction between seednode and the AddrMan

test/functional/test_runner.py

@@ -405,6 +405,7 @@ BASE_SCRIPTS = [
     'feature_shutdown.py',
     'wallet_migration.py',
     'p2p_ibd_txrelay.py',
+    'p2p_seednode.py',
     # Don't append tests at the end to avoid merge conflicts
     # Put them in a random line within the section that fits their approximate run-time
 ]