Trac #33884: use a smaller random test in edge_disjoint_spanning_trees

As discussed in https://trac.sagemath.org/ticket/32169#comment:49, we
reduce the number of nodes of a random graph used in a test.

URL: https://trac.sagemath.org/33884
Reported by: dcoudert
Ticket author(s): David Coudert
Reviewer(s): Travis Scrimshaw

src/sage/graphs/generic_graph.py

@@ -6574,7 +6574,7 @@ def edge_disjoint_spanning_trees(self, k, algorithm=None, root=None, solver=None
 
         Check the validity of the algorithms for undirected graphs::
 
-            sage: g = graphs.RandomGNP(30, .4)
+            sage: g = graphs.RandomGNP(12, .7)
             sage: k = Integer(g.edge_connectivity()) // 2
             sage: trees = g.edge_disjoint_spanning_trees(k, algorithm="MILP")
             sage: all(t.is_tree() for t in trees)