Check table rows content instead of database

Checking the database with methods like Activity.last does not test that
the record is present where it should be (first record of the table in
this case). In these tests there's only one record, though, so the order
doesn't matter that match.

However, calling methods like Activity.last generates a database query
after the process running the browser has been started, and this might
lead to inconsistent data.

spec/system/admin/system_emails_spec.rb

@@ -303,9 +303,16 @@ describe "System Emails" do
 
       visit admin_system_email_preview_pending_path("proposal_notification_digest")
 
-      expect(Notification.count).to equal(1)
-      expect(Activity.last.actionable_type).to eq("ProposalNotification")
+      expect(page).to have_content("Proposal B")
       expect(page).not_to have_content("Proposal A Title")
+
+      visit admin_activity_path
+
+      within first("tbody tr") do
+        expect(page).to have_content "Proposal notification"
+        expect(page).to have_content "Proposal A Title"
+        expect(page).to have_content admin.user.username
+      end
     end
 
     scenario "#send_pending" do