Refactor header_ci_component_spec.js using vue test utils

Description

The spec for the header_ci_component.vue is currently mounting the content of the component manually, these tests could be rewritten using mount or shallowMount from vue test utils:

• spec/frontend/vue_shared/components/header_ci_component_spec.js

Some tasks:

• Remove Vue.extend(headerCi)
• Use findMyElement() style methods instead of vm.$el.querySelector.
• Switch to data-testid where possible
• Use mount instead of shallowMount only when needed