Add syntax highlight for ActionMailer assertions

[workgena]

Reference: https://api.rubyonrails.org/v5.1/classes/ActionMailer/TestHelper.html

class ContactTest < ActiveSupport::TestCase
  include ActionMailer::TestHelper

  test 'sending mail' do
    assert_enqueued_emails 1 do
      ContactMailer.welcome.deliver_later
    end
  end
end

[tpope]

These are Rails specific and should go in the appropriate section (which might be a new section).

[workgena]

Move to this section

Also updated the example class ContactTest < ActiveSupport::TestCase. Mailer assertions can be used in any test-type by including the helper include ActionMailer::TestHelper, so I placed syn keyword ... to type_name('test').

PS: Pushed changes with force replacing original commit.