When I run:
  sudo dnf dist-upgrade   --releasever=28  --enablerepo=updates-testing

I get:

  Problém 3: package subscription-manager-rhsm-1.21.1-1.fc27.x86_64 requires subscription-manager-rhsm-certificates = 1.21.1-1.fc27, but none of the providers can be installed
  - subscription-manager-rhsm-certificates-1.21.1-1.fc27.x86_64 does not belong to a distupgrade repository
  - problem with installed package subscription-manager-rhsm-1.21.1-1.fc27.x86_64

This is because subscription-manager-rhsm no longer exist. But you cannot just stop building the package. Either one of your package should obsolete it or ask maintainer of package `fedora-obsolete` to add Obsoletes of your package.

See
https://fedoraproject.org/wiki/Upgrade_paths_%E2%80%94_renaming_or_splitting_packages