Clean unnecessary 'attachToDocument' in 'spec/frontend/issuable'

Summary

As described in &2419 (closed), we need to clean up our Jest tests from unnecessary attachToDocument usage

Risks

No risks

Involved specs

• spec/frontend/issuable_suggestions/components/app_spec.js
• spec/frontend/issuable_suggestions/components/item_spec.js
• spec/frontend/issuables_list/components/issuable_spec.js
• spec/frontend/issuables_list/components/issuables_list_app_spec.js