Convert Jest tests to use VTU in 'ee/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

• ee/spec/frontend/vue_shared/security_reports/components/dast_issue_body_spec.js
• ee/spec/frontend/vue_shared/security_reports/components/sast_container_issue_body_spec.js
• ee/spec/frontend/vue_shared/security_reports/components/sast_issue_body_spec.js
• ee/spec/frontend/vue_shared/security_reports/components/split_button_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

Close #197944 (closed)