Migrate spec/javascripts/vue_shared/components/markdown/ to Jest

The following test files should be refactored from Karma to Jest:

find spec/javascripts/vue_shared/components/markdown -name '*_spec.js' -exec echo "- [ ] {}" \;

• spec/javascripts/vue_shared/components/markdown/header_spec.js: gitlab-foss!30228 (merged)
• spec/javascripts/vue_shared/components/markdown/suggestion_diff_header_spec.js
• spec/javascripts/vue_shared/components/markdown/suggestion_diff_spec.js
• spec/javascripts/vue_shared/components/markdown/field_spec.js: !20309 (merged)
• spec/javascripts/vue_shared/components/markdown/toolbar_spec.js
• spec/javascripts/vue_shared/components/markdown/suggestions_spec.js