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

What does this MR do?

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

Risks

No risks

Involved specs

• spec/frontend/vue_shared/components/code_block_spec.js
• spec/frontend/vue_shared/components/identicon_spec.js
• spec/frontend/vue_shared/components/issue/issue_warning_spec.js
• spec/frontend/vue_shared/components/markdown/suggestion_diff_spec.js
• spec/frontend/vue_shared/components/notes/placeholder_note_spec.js
• spec/frontend/vue_shared/components/notes/placeholder_system_note_spec.js
• spec/frontend/filtered_search/components/recent_searches_dropdown_content_spec.js

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Closes #197952 (closed)