Re-organise `gfm_autocomplete_spec.rb` spec

What does this MR do?

This MR organises the gfm_autocomplete_spec.rb spec into the related sections:

• autocomplete highlighting — this consists of some tests grouped together into one aggregate_failures test so that the spec doesn't need to reload the JS page for each of these tests
  • doesnt select the first item for non-assignee dropdowns test was moved here
  • selects the first item for non-assignee dropdowns if a query is entered test was moved here
  • selects the first item for assignee dropdowns test was moved here
• assignees

The first half of the file tests when the tribute_autocomplete feature flag is off, and the second half of the file tests when it is on. The second half when the flag is on does not quite have test parity with when the flag is off, but achieving test parity is not the focus of this MR (and will be done in a subsequent MR).

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• 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
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