Replace 'source/search_code.feature' spinach test with an rspec analog

What does this MR do?

This MR does these:

1. It refactors spec/features/projects/files/find_files_spec.rb spec.
2. It adds a new test to cover the search_code.feature spinach spec.
3. It removes source/search_code.feature spec

Why was this MR needed?

This is a part of spinach tests removal campaign.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

#23036 (moved)