Add E2E for personal snippet with multiple files

What does this MR do?

Adds a new end-to-end to create a personal snippet with multiple files.

See gitlab-org/quality/testcases#842 (closed) and &2829 (closed)

NB the test enables :snippet_multiple_files feature flag and cannot be run in live environments.

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