CI: A test MAY fail with NameError: uninitialized constant ActionMailer::Base

[olleolleolle]

In the test matrix, Ruby 2.5 failed (a lone failure in a build otherwise working) with this error:

  1) Rails::Generators::DecoratorGenerator the generated decorator inheritance with an ApplicationDecorator is expected to contain "class YourModelDecorator < ApplicationDecorator"

     Failure/Error: config.after { ActionMailer::Base.deliveries.clear }

     NameError:
       uninitialized constant ActionMailer::Base

Is there some sort of ordering of tests which trips this up? Randomized with seed 51507