Refactor `gfm_autocomplete_spec.rb` for consistency

What does this MR do?

This MR refactors gfm_autocomplete_spec.rb for consistency. No tests were added/removed, just refactored.

Some things done:

• Changed to use Capybara actions (e.g. fill_in, find_field) to target what the user sees rather than relying on CSS selectors which is more brittle
• Updated assertions to use methods like find_autocomplete_menu for readability and consistency
• Removed page.withins which are unnecessary

Note: This file contains tests for the feature flag tribute_autocomplete switched off and on, to test two different JS autocomplete libraries (one of which we're migrating to)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• 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
• Security reports checked/validated by a reviewer from the AppSec team