Remove unneeded resetNodes().

We do this in the init() function of Dijkstra and BFS every time
calculate(startNode) is called. There is no need to do it twice.
Put the toString() method back
Rename this test appropriately
Add GraphPrep and all possible Dijkstra tests

The new GraphPrep makes it easy to enter a new example graph and
consider all possible orientations of (un)weighted
(un)directed/reversed.

TODO: Make the DijkstraCormenUndirectedStarToStarTest use the
CormenGraphPrep.