test, Generators::GeneratedAttribute with references, required, index.

Closes #17197.
Closes #17207.

`{required}` is a type modifier so it should be:

    user:references{required}:index

and not:

    user:references:index{required}

railties/test/generators/generated_attribute_test.rb

@@ -141,4 +141,12 @@ def test_handles_column_names_for_references
     assert_equal "post_id", create_generated_attribute('references', 'post').column_name
     assert_equal "post_id", create_generated_attribute('belongs_to', 'post').column_name
   end
+
+  def test_parse_required_attribute_with_index
+    att = Rails::Generators::GeneratedAttribute.parse("supplier:references{required}:index")
+    assert_equal "supplier", att.name
+    assert_equal :references, att.type
+    assert att.has_index?
+    assert att.required?
+  end
 end