Fix poll_spec transient failures

What does this MR do?

Removes the use of vue-resource as a mock service. Uses a mock service that is just a spy object with a fetch method that will instantly return a "response", rather than going through vue-resource and its interceptors yada, yada.

Also removes the setTimout magic 5 which is never a good sign. 😛

https://gitlab.com/gitlab-org/gitlab-ce/builds/16010974

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

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