Fix flaky web push test and deduplicate DNS stub helper (#2825)

* Extract shared DnsTestHelper from duplicated stub_dns_resolution methods

* Fix flaky WebTest by stubbing DNS resolution for push subscription creation

test/controllers/api/flat_json_params_test.rb

@@ -247,11 +247,4 @@ class FlatJsonParamsTest < ActionDispatch::IntegrationTest
     assert_response :no_content
     assert_equal "Flat Join", users(:david).reload.name
   end
-
-  private
-    def stub_dns_resolution(*ips)
-      dns_mock = mock("dns")
-      dns_mock.stubs(:each_address).multiple_yields(*ips)
-      Resolv::DNS.stubs(:open).yields(dns_mock)
-    end
 end

test/controllers/users/push_subscriptions_controller_test.rb

@@ -92,11 +92,4 @@ class Users::PushSubscriptionsControllerTest < ActionDispatch::IntegrationTest
 
     assert_response :unprocessable_entity
   end
-
-  private
-    def stub_dns_resolution(*ips)
-      dns_mock = mock("dns")
-      dns_mock.stubs(:each_address).multiple_yields(*ips)
-      Resolv::DNS.stubs(:open).yields(dns_mock)
-    end
 end