Modify create_personal_snippet_spec to verify code highlighting

What does this MR do?

Modifies create_personal_snippet_spec.rb to create a snippet with Ruby code and check that syntax highlighting is present.

Addresses a test gap - snippets usually contain code, however most of our tests work with plain text snippets.

To run the test:

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

Closes gitlab-org/quality/testcases#899 (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