Move `test_skip_bundle` to `AppGeneratorTest`

`skip_bundle` option was removed from plugin generator in 9b72fcc3.

railties/test/generators/app_generator_test.rb

@@ -105,6 +105,15 @@ def default_files
     ::DEFAULT_APP_FILES
   end
 
+  def test_skip_bundle
+    assert_not_called(generator([destination_root], skip_bundle: true), :bundle_command) do
+      quietly { generator.invoke_all }
+      # skip_bundle is only about running bundle install, ensure the Gemfile is still
+      # generated.
+      assert_file "Gemfile"
+    end
+  end
+
   def test_assets
     run_generator
 

railties/test/generators/shared_generator_tests.rb

@@ -103,15 +103,6 @@ def test_skip_gemfile
     end
   end
 
-  def test_skip_bundle
-    assert_not_called(generator([destination_root], skip_bundle: true), :bundle_command) do
-      quietly { generator.invoke_all }
-      # skip_bundle is only about running bundle install, ensure the Gemfile is still
-      # generated.
-      assert_file "Gemfile"
-    end
-  end
-
   def test_skip_git
     run_generator [destination_root, "--skip-git", "--full"]
     assert_no_file(".gitignore")