Improve flaky snippet_index_page_spec test

What does this MR do?

It addresses an expected delay in snippet statistics creation and computation before the number of files appears on the Index page. The current lack of wait (and, what is more important, no page reload) causes the snippet_index_page_spec test to fail intermittently on some environments. See #338313 (closed) for the background of the issue.

Does this MR meet the acceptance criteria?

Conformity

• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.