askrene: don't skip fee_fallback test

The fee_fallback test would fail after fixing the computation of the
median. Now by we can restore it by making the probability cost factor
1000x higher than the ratio of the median. This shows how hard it is to
combine fee and probability costs and why is the current approach so
fragile.

Changelog-None

Signed-off-by: Lagrang3 <[email protected]>

Author: Lagrang3

plugins/askrene/mcf.c

@@ -482,7 +482,7 @@ static void combine_cost_function(
 {
 	/* probabilty and fee costs are not directly comparable!
 	 * Scale by ratio of (positive) medians. */
-	const double k = get_median_ratio(working_ctx, linear_network);
+	const double k = 1000 * get_median_ratio(working_ctx, linear_network);
 	const double ln_30 = log(30);
 	const struct graph *graph = linear_network->graph;
 	const size_t max_num_arcs = graph_max_num_arcs(graph);

tests/test_askrene.py

@@ -552,7 +552,6 @@ def test_getroutes(node_factory):
                             'delay': 99 + 6}]])
 
 
-@pytest.mark.skip
 def test_getroutes_fee_fallback(node_factory):
     """Test getroutes call takes into account fees, if excessive"""