* Run libraries package testing on build agent

instead of on Helix as with recent changes the entire package testing
doesn't take more than 2 minutes. Helix created a work item per package
test even though it only took seconds and the average wait time for
a client was 10-15min.

Also cleaning up how the generated package test projects are restored
and incorporating a fix from Eric St John to not hard code the package
feeds.

* Expose useHelix argument