[test] Test the add_outbound_p2p_connection functionality

Open max number of full-relay and block-relay-only connections from a
functional test with different sorts of behaviors to ensure it behaves as
expected.

test/functional/test_runner.py

@@ -261,6 +261,7 @@ BASE_SCRIPTS = [
     'feature_filelock.py',
     'feature_loadblock.py',
     'p2p_dos_header_tree.py',
+    'p2p_add_connections.py',
     'p2p_unrequested_blocks.py',
     'p2p_blockfilters.py',
     'feature_includeconf.py',