Add E2E to test deleting a file from snippet

What does this MR do?

Adds an E2E to delete one file from a personal/project snippet and make it single-file. Adds coverage for recently added Snippets with Multiple Files feature.

It also modifies snippet creation via API to allow for multi-file snippets.

Closes gitlab-org/quality/testcases#1023 (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