`feedback_arc_set()` needs tests

[szhorvat]

Guidelines:

• Test that removing the returned edges results in an acyclic graph (is_acyclic())
• Test directed an undirected separately
• When using exact methods, test the size of the feedback arc set. There are multiple valid solutions, but all should have the same size (or total weight)
• When using exact methods, stick to tiny graphs of at most tens of edges, otherwise the computation is too slow.
• Exact methods are only available when igraph is compiled with GLPK. See testthat/test-optimal.community.R for how to deal with this.

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