DEV: Fix flaky "Changing email" system tests (#25805)

Why this change?

`current_url` does not rely on Capybara waiters so opt to use
`have_current_path` matcher instead. Also assert for email against
element displayed on the page instead of querying the database for it
which isn't really what system tests are meant for.

spec/system/page_objects/pages/user_preferences.rb

@@ -41,6 +41,10 @@ module PageObjects
         horizontal_secondary_link_visible?(ACCOUNT_LINK_CSS_SELECTOR, visible: false)
       end
 
+      def has_primary_email?(email)
+        has_css?(".email-first", text: email)
+      end
+
       private
 
       def horizontal_secondary_link_visible?(selector, visible: true)