Ensure unit and integration tests run for any change to our central Gitaly test setup file [RUN ALL RSPEC]

This moves scripts/gitaly_test.rb to spec/support/helpers/gitaly_setup.rb.

This is done for two reasons:

1. It makes more sense to have this file under spec/support/helpers rather than under scripts since it's used both locally (through spec/support/helpers/test_env.rb -> scripts/gitaly-test-spawn -> spec/support/helpers/gitaly_setup.rb) and on the CI (through scripts/gitaly-test-spawn -> spec/support/helpers/gitaly_setup.rb).
2. The {,ee/}spec/**/*.rb pattern is part of the "backend" patterns which is used to decide if unit and integration jobs need to run.

Background

We had a broken master because unit and integration tests didn't run on #327591 (closed) which only updated scripts/gitaly_test.rb, which is a central piece of our code base.

This will prevent such cases in the future.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because ~"tooling::pipelines".
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Related to #327608 (closed)