Update tests for Dijkstra's algorithm to test both graph models

Use the already established macro pattern, move actual tests to a new file in src/testing.
Use the make_test_graph() function from the graph topology tests instead of duplicating it for the Dijkstra tests.

src/testing.rs

@@ -1,2 +1,3 @@
+pub(crate) mod dijkstra_testing;
 pub(crate) mod graph_topology_testing;
 pub(crate) mod maps_testing;