Test app task: Allow passing in user class

Without this, every extension's test app always has `Spree::LegacyUser`
configured. Mostly, that's fine, but for solidus_auth_devise, it'd be
nicer if the generated spree.rb file would have `Spree.user_class =
"Spree::User"` rather than `Spree::LegacyUser`.

core/lib/spree/testing_support/extension_rake.rb

@@ -4,8 +4,8 @@
 
 desc "Generates a dummy app for testing an extension"
 namespace :extension do
-  task :test_app, [:user_class] do |_t, _args|
+  task :test_app, [:user_class] do |_t, args|
     Spree::DummyGeneratorHelper.inject_extension_requirements = true
-    Rake::Task['common:test_app'].invoke
+    Rake::Task['common:test_app'].invoke(args[:user_class])
   end
 end