E2E for adding comments to snippets

What does this MR do?

Creates a new end-to-end test for adding comments on personal and project snippets. This has been identified as a test gap and the new test will ensure we have comprehensive coverage for snippets.

To run the test:

CHROME_HEADLESS=false QA_DEBUG=true bundle exec bin/qa Test::Instance::All http://localhost:3000 -- qa/specs/features/browser_ui/3_create/snippet/add_comment_to_snippet_spec.rb

Closes gitlab-org/quality/testcases#814 (closed)

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