Add test for the mail package.
Add test for the mail package.

Change-Id: Ibf069ccf1e6682fff947707a451d9f808538974e

golang/appengine,flowlo/appengine,flowlo/appengine,skyeunknown084/appengine,mzimmerman/appengine,flowlo/appengine,fuchsia-mirror/third_party-github.com-golang-appengine,skyeunknown084/appengine,mzimmerman/appengine,fuchsia-mirror/third_party-github.com-golang-appengine,mzimmerman/appengine,skyeunknown084/appengine,golang/appengine