Replace js-dropdown-input-field with data testid attribute

.js-dropdown-input-field is the css class used to select the input element for testing. Per our guideline, we should replace it with a data testid attribute. https://docs.gitlab.com/ee/development/testing_guide/frontend_testing.html#how-to-query-dom-elements.

Replace the css class in the following files:

• user_select.vue
• assignee_select.vue
• issue_sidebar_spec.rb