Description of problem: It would be able to be able to easily enable the appropriate testing repos (updates-testing for Fedora, epel-testing for EPEL) for a copr and/or individual builds.
In project settings, you can enable external repositories. So you can just insert the appropriate repository _baseurl_ there. For fedora-updates-testing that would be: http://download.fedoraproject.org/pub/fedora/linux/updates/testing/$releasever/$basearch/ For epel7: http://download.fedoraproject.org/pub/epel/testing/7/$basearch
I am closing as the current situation seems to be sufficient.