Create property-based tests for shortest paths algorithms

[pnevyk]

Create proptest tests for ShortestPaths using our graph generation ~strategies~ strategies. Test both algorithms (Dijkstra, Bellman-Ford). ~Use DebugGraph as the graph being generated because it provides very useful information in case of a failed assert.~

What properties to test? ~One possibility is to implement a function for removing isolated vertices from a graph so that the tested graphs are connected. Then,~ For connected graphs after finishing an algorithm, it should be possible to reconstruct path and get distance from every vertex in the graph to the starting vertex. Also, the distances (not the paths, generally) should be equal as returned by Dijkstra and Bellman-Ford.

[pnevyk]

Update after #46:

• The proptests should look similarly to those for toposort https://github.com/pnevyk/gryf/blob/4518875bcaaa296b5ccfa4b0e6943a9fb5dfd455/src/algo/toposort.rs#L269-L297.
• It is possible to request for generated graphs to be connected, but this feature is not implemented yet https://github.com/pnevyk/gryf/blob/4518875bcaaa296b5ccfa4b0e6943a9fb5dfd455/src/infra/proptest.rs#L322-L327 (might be a part of this ticket).

[pnevyk]

In #64 I added proptests that at least check that the algorithms do not panic. But the properties mentioned in the original post in this issue should still be added.