Fix ivar name (#219)

Found in [ruby
2.7](https://github.com/kachick/ruby-gem-template/actions/runs/3179027537/jobs/5181100027),
it makes warnings `warning: instance variable @cached_parent_test not
initialized`

Looks related to 3058e8a

lib/test/unit/attribute.rb

@@ -199,7 +199,7 @@ def find_attribute(method_name, name, options={})
  ancestor.is_a?(Class) and
  ancestor < Test::Unit::Attribute
  end
- return nil if @cached_parent_test.nil?
+ return nil if @cached_parent_test_case.nil?
 
  @cached_parent_test_case.find_attribute(method_name, name, options)
  end