Fix unit tests (properly mirror CRAN repo structure)

Archived packages are stored in the format e.g.

http://<cran mirror>/src/contrib/Archive/<package>/<package>_<version>.tar.gz

while current versions of packages are stored as

http://<cran mirror>/src/contrib/<package>_<version>.tar.gz

Previously, the unit test repository was structured such that current versions of test packages were stored in e.g.

http://<cran mirror>/src/contrib/<package>/<package>_<version>.tar.gz