Prepare markdown-header spec for bv upgrade

What does this MR do?

This MR lays the groundwork for the BootstrapVue upgrade that will be done in this MR: !18913 (merged) Due to some changes in the BTooltip and BPopover components, some related tests started failing after the upgrade. Those tests need to be updated accordingly:

• Migrate specs to use vue-test-utils
• sync needs to be set to false
• attachToDocument must be true

This MR focuses on upgrading the tests located in spec/frontend/vue_shared/components/markdown/header_spec.js.

Screenshots

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team