--promote-failures: reroute with the connections that failed moved first

Each round routes every connection that has failed so far ahead of the
rest, which keep their order. The promoted set only grows, so two groups
cannot take turns failing; the best round is kept.

tests/test_cli.py

@@ -156,6 +156,7 @@ def test_cli_supported_combinations_are_accepted(tmp_path):
         "max_passes": ["-mp", "4"],
         "seed": ["--seed", "3"],
         "orders": ["--orders", "2"],
+        "promote": ["--promote-failures", "2"],
     }
     for engine, options in ENGINE_OPTIONS.items():
         for option in options: