This is wrong, you have to load your package explicitly.

From http://cran.r-project.org/doc/manuals/R-exts.html#Package-subdirectories:
"The directory tests is copied to the check area, and the tests are run
with the copy as the working directory and with R_LIBS set to ensure
that the copy of the package installed during testing will be found by
library(pkg_name)."