Test-specific dependencies?

Is CompatHelper supposed to also work for dependencies that are not required by a package, but are required to test the package, a.k.a. test-specific dependencies? [DSP.jl recently ran into a situation](https://github.com/JuliaDSP/DSP.jl/pull/375) where its test-specific dependency compat range was not being expanded by CompatHelper, which eventually caused [test errors](https://github.com/JuliaDSP/DSP.jl/issues/373). I am not sure if this was due to misconfiguration on our part, or if this is an issue with CompatHelper. The test-specific dependency was specified using [the method of specifying test dependencies that works for Julia 1.0 and 1.1](https://julialang.github.io/Pkg.jl/v1/creating-packages/#Test-specific-dependencies-in-Julia-1.0-and-1.1-1), so that we can support the LTS version of Julia.