test: Test handling of "return.vs.es" in several functions

[stibu81]

This completes the fix of #1605 and adds tests.

Since the tests of neighbors() were based on as_adj_list(), I have extended the test of the latter to also run with return.vs.es = FALSE. I extended the tests of neighbors() and added tests for adjacent_vertices() and incident_edges().

I found another small issue related to this one that I think is a mistake. But I am not entirely sure that it is, so I would like to have confirmation before I fix it: as_adj_edge_list() does not respect the setting return.vs.es and always returns igraph.es objects. Could someone please confirm that this is indeed not the expected behaviour. I could then fix this and update the pull request. Or should I open a new issue for this?

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[krlmlr]

Thanks a lot, just in time for the release!

[stibu81]

Could you comment on the other issue I found? It could be fixed quickly if you agree that it is a mistake.

[krlmlr]

Thanks, good catch, missed that. This is a new issue and a potentially breaking change, let's handle this separately.

[stibu81]

Should I open an issue where this can be discussed?

[krlmlr]

Please do, thank you. Chances are that other functions are affected here.