Add sleep call before yield in allowing_for_delay

This is to give pending AJAX requests time to complete before we
navigate away, for example.

spec/support/capybara_helpers.rb

@@ -14,6 +14,8 @@ module CapybaraHelpers
     tries = 0
     begin
+      sleep interval
+
       yield
     rescue Capybara::ExpectationNotMet => ex
       if tries <= retries