Before it was in AdjacencyListGraphReadertTest, now it is shared only by the 2 readers tests in that package. I find it rather local and didn't want a special file for this. And, by the way, I couldn't find a fitting place for this.

If you like, I can put it in either:
1) main/util/GraphMapEquality.scala
2) test/util/GraphMapEquality.scala
3) main/util/io/GraphMapEquality.scala
4) test/util/io/GraphMapEquality.scala

or somewhere in existing file.