Unit testing seems to be more or less a solved problem, so all three packages will likely be adequate for your needs. There are subtle differences between them though:

RUnit is based on xunit, and as such is easy to understand if you’ve used any versions of it from other languages.

svUnit uses the same tests as RUnit, but includes a GUI to help with interactive use.

testthat isn’t compatible with either, but includes much the same features and can check to only execute tests on files that haven’t changed, which is useful for testing large projects.