qa: Limit -maxconnections in tests

On systems such as NetBSD, this change enables the execution of
functional tests.

test/functional/test_framework/test_node.py

@@ -117,6 +117,9 @@ def __init__(self, i, datadir_path, *, chain, rpchost, timewait, timeout_factor,
             "-debugexclude=leveldb",
             "-debugexclude=rand",
             "-uacomment=testnode%d" % i,  # required for subversion uniqueness across peers
+            # Limit max connections to mitigate failures on some systems caused by the warning:
+            # "Warning: Reducing -maxconnections from 125 to <N> due to system limitations".
+            f"-maxconnections={MAX_NODES}",
         ]
         if self.descriptors is None:
             self.args.append("-disablewallet")