Test error on 1.6 beta

[mfalt]

Doctest, and therefore tests fail on PkgEval for the upcoming Julia 1.6. It is a bit annoying that we keep getting errors from formatting of the printing, but it was also bad before we tested against the docs when the documentation got outdated. A quick fix would be to ignore white space in the doctest at least. See log here:
https://s3.amazonaws.com/julialang-reports/nanosoldier/pkgeval/by_hash/56ff37a_vs_ebcc603/ControlSystems.1.6.0-beta1-73f862b30f.log

Edit: We should probably also not hard-code the types in the regular tests, but instead do something like described in https://discourse.julialang.org/t/analysis-of-pkgeval-results-for-upcoming-1-6/53194
e.g.
'"2-element $(Array{BenchmarkTools.TrialEstimate,1}):\n 0.490 ns\n 1.000 ns"