Move 'Search Snippets' Spinach feature to Rspec

## What does this MR do?
Moves the Spinach feature `features/snippet_search.feature` to an RSpec feature `spec/features/snippets/search_snippets_spec.rb`.

## Are there points in the code the reviewer needs to double check?

The search results page URL was being visited directly in the original Spinach test, using an encoded search query and specific parameters to trigger the snippet searches. I have changed the RSpect feature to fill in the search box on the snippets dashboard page to retrieve results to cover the missing test coverage on the search box and to avoid the feature test from relying on the implementation details of the search URL structure.

## Why was this MR needed?

As part of deprecating the Spinach test suite.

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [-] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [-] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [-] API support added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

#23036

See merge request !7494