Further stability improvement on the Flatpickr spec

Author: mshibuya

spec/integration/widgets/datetimepicker_spec.rb

@@ -21,6 +21,7 @@
 
   it 'populates the value selected by the Datetime picker into the hidden_field' do
     visit new_path(model_name: 'field_test')
+    is_expected.to have_css '.form-control.flatpickr-input', visible: false
     page.execute_script <<-JS
       document.querySelector('#field_test_datetime_field')._flatpickr.setDate('2015-10-08 14:00:00');
     JS
@@ -54,7 +55,7 @@
       end
     end
     visit new_path(model_name: 'field_test')
-    is_expected.to have_text 'New Field test'
+    is_expected.to have_css '.form-control.flatpickr-input', visible: false
     page.execute_script <<-JS
       document.querySelector('#field_test_datetime_field')._flatpickr.setDate('2015-10-08 12:34:56');
     JS