Update spec expectations to avoid flake specs

The last expectation we were using in this test is satisfied before
going back to the admin stats page, as the campaing2 name is not
present before clicking the `Go back` link. Because of this, the
test could end while the request thrown by the `Go back` link is
not completed yet, which can collide with the following test and
cause a flake spec.

spec/system/campaigns_spec.rb

@@ -24,12 +24,12 @@ describe "Email campaigns", :admin do
     visit root_path(track_id: Campaign.last.id + 1)
 
     visit admin_stats_path
+
+    expect(page).to have_content campaign1.name
+    expect(page).not_to have_content campaign2.name
+
     click_link campaign1.name
 
     expect(page).to have_content "#{campaign1.name} (1)"
-
-    click_link "Go back"
-
-    expect(page).not_to have_content campaign2.name.to_s
   end
 end