Some of the tests use the attributes method which return a hash of all the attributes in the db.
As a result after adding a plugin and removing it without getting rid of the columns that were added to host you get:

HostsControllerTest#test_0072_template_used returns templates with interfaces:
ActiveModel::MassAssignmentSecurity::Error: Can't mass-assign protected attributes for Host::Managed: salt_proxy_id, salt_environment_id
    app/controllers/hosts_controller.rb:606:in `template_used'
    app/controllers/concerns/application_shared.rb:13:in `set_timezone'
    app/models/concerns/foreman/thread_session.rb:32:in `clear_thread'
    test/functional/hosts_controller_test.rb:953:in `block in <class:HostsControllerTest>'

The tests should only try and assign the attributes that belong to Host::Managed.