Migrate spec/javascripts/ci_variable_list/ to Jest

Description

Let's migrate the specs in spec/javascripts/ci_variable_list/ from Karma to Jest.

See the related epic for how to do this.

• spec/javascripts/ci_variable_list//ajax_variable_list_spec.js
• spec/javascripts/ci_variable_list//native_form_variable_list_spec.js
• spec/javascripts/ci_variable_list//ci_variable_list_spec.js