Convert Jest tests to use VTU in 'spec/frontend/groups'

Summary

As described in &2445 (closed), we need to convert "old style" test suites to use @vue/test-utils

Involved specs

• spec/frontend/groups/components/app_spec.js
• spec/frontend/groups/components/group_folder_spec.js
• spec/frontend/groups/components/group_item_spec.js
• spec/frontend/groups/components/groups_spec.js
• spec/frontend/groups/components/item_actions_spec.js
• spec/frontend/groups/components/item_caret_spec.js
• spec/frontend/groups/components/item_stats_spec.js
• spec/frontend/groups/components/item_stats_value_spec.js
• spec/frontend/groups/components/item_type_icon_spec.js