Enable test_existing_database after Katello fixes

Katello/katello@07fdfc3
was the last fix required for this to work

Author: evgeni

.github/workflows/unit_tests.yaml

@@ -21,4 +21,3 @@ jobs:
     uses: theforeman/actions/.github/workflows/foreman_plugin.yml@v0
     with:
       plugin: foreman_kernel_care
-      test_existing_database: false