[E2E] PROMOTE TO BLOCKING: add_file_to_snippet_spec.rb

What does this MR do?

Promotes the following e2e tests to the blocking bucket:

1. Create Multiple file snippet behaves like adding file to snippet adds second file to an existing project_snippet to make it multi-file | Testcase | Spec metrics

This MR was created based on data from reliable e2e test report: #457423 (closed)

(This MR was automatically generated by gitlab_quality-test_tooling at 2024-04-22 02:05:46 UTC)