Extract the hostip.info stubbing into a common method.

There doesn't appear to be any before(:suite) equivalent for Minitest
so this is the least-surprising way to do this.

test/controllers/site_controller_test.rb

@@ -9,7 +9,7 @@ class SiteControllerTest < ActionController::TestCase
     Object.const_set("ID_KEY", client_applications(:oauth_web_app).key)
     Object.const_set("POTLATCH2_KEY", client_applications(:oauth_web_app).key)
 
-    stub_request(:get, "http://api.hostip.info/country.php?ip=0.0.0.0")
+    stub_signup_requests
   end
 
   ##

test/controllers/user_controller_test.rb

@@ -4,7 +4,7 @@ class UserControllerTest < ActionController::TestCase
   api_fixtures
 
   setup do
-    stub_request(:get, "http://api.hostip.info/country.php?ip=0.0.0.0")
+    stub_signup_requests
   end
 
   ##