Add ElementMap::reserve to replace ElementMap::expand, update tests

ElementMap tests are now self contained unit tests, without any graph context. The only exception is the repurposed test "reused_slot_returns_old_value", which is not suited for GraphTopologyDeletion, but actually specific for Graph.

src/testing.rs

@@ -4,4 +4,3 @@ pub(crate) mod bfs_testing;
 pub(crate) mod dfs_testing;
 pub(crate) mod dijkstra_testing;
 pub(crate) mod graph_topology_testing;
-pub(crate) mod maps_testing;