GraphPlot needs more tests

[simonschoelly]

There are large parts of the code that are not covered by tests. Furthermore, there should be tests special cases, such as graphs with 0, 1, 2 or three vertices, few edges, self-loops and so on as some of the layouts have problems with these cases.

[sbromberger]

Closed by #66 and JuliaLang/METADATA.jl#18449

[simonschoelly]

I think this should still be open, at the moment GraphPlot has only 4 tests in total and doesn't cover much of the code.

[dietercastel]

My 2 cents in this PR