test: add tests for graph_from_biadjacency_matrix()

[maelle]

Also characterization tests as prep for #1518

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[maelle]

Based on the coverage results https://app.codecov.io/gh/igraph/rigraph/pull/1520/blob/R/incidence.R

I need to add weights to the cases that I thought would cover the modes for graph.incidence.sparse().

For graph.incidence.dense()... I am confused why the different modes aren't covered, I'll have to step into the function.

[maelle]

@szhorvat do these characterization tests look ok to you? (needed for #1518)

[szhorvat]

weighted=T and multiple=T should be mutually exclusive. We either interpret numbers larger than 1 as weights or as multiplicities, but it cannot be both.

I suggest adding an error when setting both to true.

This is not an issue in the C core, as we have separate functions for the unweighted and weighted cases, and only the unweighted function has a multiple parameter. But in R we have a single function (and we should keep it that way).

[szhorvat]

Same comment about weights as before. Have a diversity of weights. Have at least two different kinds of weights (sample(0:2)), but ideally have non-integer weights as well. It's a question of how much time you want to dedicate to testing. There are lots of cases.

[maelle]

#1544