This fails tools:::.check_packages_used()$imp2un. The comment appears misguided -- I find no evidence this function has ever existed. In particular here's the {parallel} NAMESPACE for R 3.0.2:

https://github.com/wch/r-source/blob/tags/R-3-0-2/src/library/parallel/NAMESPACE

Checked the full SVN history with a script from Gemini:

https://gist.github.com/MichaelChirico/2f5e578c3464ea584bd4a006563ecd94

So I think we can simplify this code by just assuming the test will fail.