Resolve "`bin/rspec`/`bin/karma` can nuke the GDK entirely"

What does this MR do?

This implements 3 level of safety to ensure we don't accidentally nuke the whole GDK installation dir.

1. Ensure RAILS_ENV is set to test in spec/spec_helper.rb
2. Abort TestEnv.init if RAILS_ENV is not test
3. Ensure the Gitaly/GiLab Shell install dirs are well named (so that if the path is somehow wrong, e.g. gdk, we abort and don't accidentally remove a wrong install dir).

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

I've refactored the Gitaly & GitLab Shell setup methods, they still work after rm -rf tmp/tests, though.

Does this MR meet the acceptance criteria?

• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #37872 (closed)