Add cut/separator tests

[vtraag]

We need tests that double check cut results. In particular, tests should check (1) whether a cut is indeed an incorrect cut (i.e. after removing the proposed edges/vertices in the cut/separators, we should double check the connectivity); and (2) for minimal cuts, we should check whether they are indeed minimal.

In particular, the graphs listed in #2532 could be tested for cuts, while the graphs listed in #2521 could be used for separators.

One could use igraph_distances to check whether there is a path still between two specific nodes. So, the core of the test could be something like the following for cuts (and something similar for separators).

igraph_copy(&subgraph, &graph);
igraph_delete_edges(&subgraph, igraph_ess_vector(cut));
igraph_distances(&subgraph,
                &res,
                igraph_vss_1(source),
                igraph_vss_1(target),
                IGRAPH_OUT);
igraph_bool_t is_cut = !isinf(MATRIX(res, 0, 1));

[ijbarnes]

Can I work on this? Im looking to contribute to an open source project for the first time.

[vtraag]

Yes, you are also welcome to work on this!

Before getting started, make sure that you are able to compile a simple test program yourself, see https://igraph.org/c/doc/igraph-Tutorial.html to get started (see https://igraph.org/c/doc/igraph-Installation.html for how to install the development version of igraph).

One you've passed those first hurdles, try to see if you can create a small program according to the specifications above. Once you've got something running, it would be good to open a (draf) PR. Once it all works as expected we can integrate it properly into igraph as a test, I'll help you on your way there.

[szhorvat]

Just to be aware that @ijbarnes also asked about #2511. Be sure to work on one issue at a time, and let us know which one you chose.