Improve wrapper test for c82574

This test is able to cover changes from c82574.

test/form_builder/wrapper_test.rb

@@ -60,12 +60,10 @@ class WrapperTest < ActionView::TestCase
   end
 
   test 'wrapper does not determine if valid class is needed when it is set to nil' do
-    def @user.name; raise BOOM; end
-    with_form_for @user, :name, as: :file, wrapper: custom_wrapper_with_input_valid_class(valid_class: nil)
-    assert_select 'div'
-    assert_select 'input'
-    assert_no_select 'div.field_with_errors'
-    assert_no_select 'input.is-invalid'
+    @user.instance_eval { undef errors }
+    with_form_for @user, :name, wrapper: custom_wrapper_with_input_valid_class(valid_class: nil)
+
+    assert_no_select 'div.field_without_errors'
   end
 
   test 'wrapper adds hint class for attribute with a hint' do