clarify that run-all.R goes under R/tests, not inst/tests

When mentioned right after inst/tests, tests/run-all.R looks like inst/tests/run-all.R, at least to me when I'm learning this stuff. I tracked down [R CMD check docs](http://cran.r-project.org/doc/manuals/R-exts.html#Checking-packages) and realized my error. Being just a little more explicit might have helped me understand in the first place.