Speed up runners API specs

What does this MR do?

This MR improves the spec performance of Runners API specs by using let_it_be and before_all (❗) by 3 minutes (❗).

Before

Finished in 3 minutes 33.1 seconds (files took 34.01 seconds to load)
119 examples, 0 failures

After

Finished in 33.18 seconds (files took 1.06 seconds to load)
119 examples, 0 failures

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team