Improve the test suite

[tomasaschan]

While reading up on old julia-users posts, I found this thread which announces IVPTestSuite.jl. ODE.jl solvers are tested in the benchmarks published in that thread, but we could probably utilize this package to better our own test suite as well.

I haven't had time to look into it (and I don't know when I will have, since I don't currently do any work related to ODE solving) but I think it would be a shame if the possibility was forgotten, so I'm leaving this here for whenever I or someone else has time to tackle it.