Debugging 'testthat' tests in RStudio

Is it possible to invoke the debugger in RStudio when running testthat tests? I haven't been able to find a setup that allows this (various combinations of "use devtools package functions if available" in the settings, hitting the "Test Package" option in the "Build -> More" menu, running test() in the console, putting in browser() calls, etc.) but haven't found a way yet.

I also find myself getting lost a lot when testing, unsure whether the code being run has been installed in the system libraries (by doing 'Build & Reload'), or is being run in situ from the local R directory, or what - sometimes RStudio complains that a breakpoint can't be set until the package is rebuilt (so I suspect the former) or doesn't (so I suspect the latter). Not sure if this issue is closely related or not to my main question.

Without finding a way to drop into the debugger, I end up pasting test code into the console & working in a very ad-hoc fashion, and basically shooting my TDD habits in the foot. So any advice would be appreciated - if it's not possible to invoke the debugger, any suggested workarounds?

I'm running RStudio version 0.99.447 on OS X, in local mode, with R 3.2.1.