Add Karma tests for create_item_dropdown.js

Add Karma tests for create_item_dropdown.js

create_item_dropdown.js was renamed from protected_branch_dropdown/protected_tag_dropdown in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4043

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protected_branch_crate/protected_branch_tag should also have specs but there are rspec tests in spec/features/protected_tags_spec.rb/spec/features/protected_tags_spec.rb.