This is a pretty important new feature in the test suite, which
is to construct a remote repository on the fly as part of the
test suite.

The general principle is that you create a directory full of folders
for all of the packages you want available in the repo, and then
the 'withRepo' function will initialize this into a secure repo
you can do tests with.

Fixes #4016.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>