Fix namespace problem with dynamic vuex import

What does this MR do?

• Fixes the Vue instances initialization for issuable_lock_form.vue

  • After we started using dynamic import, we had to pass in store.default rather than just store in mount_sidebar.js#L145. In this MR, we instead return store.default in the promise returned by the dynamic import.

• Adds a feature spec to allow us to detect problems earlier.

Issue #255907 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

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

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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