Improve readability of vulnerability header_spec

What does this MR do and why?

No new tests were added.

I reorganized the spec file to improve its readability. Reading the spec file now, it should be easier to identify which components of the header.vue file each portion of the tests is testing. More or less, it matches like so:

In comparison to before, there were some straggling test blocks that were tougher to pinpoint which vue component it was actually testing:

The flow is like this now:

• resolution alert when the vulnerability is no longer detected on the default branch
• status description
  • vulnerability user watcher tests moved here
• state dropdown changes
• action buttons

Screenshots or screen recordings

No UI/UX changes

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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