Bug#1140569: unblock: R packages (test-)installable on only some architectures

[Rebecca N. Palmer]

It's also quite likely that the packages are installable on the affected 
architectures but their test-Depends aren't: none of them list 
installability problems on the excuses page itself (as opposed to the 
autopkgtest failure logs), and the uninstallable test-dependency is 
often r-cran-covr, a test coverage checker and hence probably not a 
runtime Depends.

If they don't migrate soon, I'll investigate further.