First some tests where us and them are equal:
No vertices:
( )

One vertex:
( 0 )

Directed disconnected graph with loops and multiple edges, no weights, k = 0, IGRAPH_IN:
( 1 2 1 2 2 0 )

Same graph, with weights:
( 0.3 0 0 0.6 1.1 0 )

Same graph, k=1:
( 2 2 2 3 2 0 )

Same graph, k=1, IGRAPH_ALL:
( 4 3 3 5 2 0 )

Same graph, without loops, k=1:
( 2 1 2 2 2 0 )

Same graph with loop, k=1, undirected:
( 4 3 3 5 2 0 )

Same graph, directed, k=2:
( 2 4 2 6 5 0 )

Same graph, undirected, k=2:
( 6 8 8 8 5 0 )

Same graph, weighted:
( 0.9 2 2 2 1.8 0 )

Now some tests where us and them are not equal:
Us = same graph, them = edgless, directed, k=1, should show 0, because there are no edges:
( 0 0 0 0 0 0 )

Switched us and them, should show only 1 at the loop:
( 0 1 0 0 0 0 )

Us = same graph, them = only edge from 0 to 1, directed, k=1, IGRAPH_IN, should show edge from 0 to 1:
( 0 1 0 0 0 0 )

Switched us and them, should show edge from 0 to 1 and loop:
( 0 2 0 0 0 0 )

Us = same graph, them = full graph, directed, k=3, IGRAPH_ALL, should show 4*5=20 edges for the connected part:
( 20 20 20 20 20 0 )

Switched us and them, should show 8 edges for the whole graph:
( 8 8 8 8 8 8 )

Check error handling:
Wrong size weights.
Negative k.
Number of vertices in us and them not equal.
Directedness in us and them not equal.
