Strategy spec speedup

What does this MR do?

A small fixup to our strategy spec to make it faster while reducing the lines of code. Followup from this comment: !43340 (comment 422315532)

Baseline:

± bin/rspec spec/models/operations/feature_flags/strategy_spec.rb
Run options: include {:focus=>true}

All examples were filtered out; ignoring {:focus=>true}
[...]

Finished in 8.87 seconds (files took 38.03 seconds to load)
217 examples, 0 failures

Using let_it_be:

± bin/rspec spec/models/operations/feature_flags/strategy_spec.rb
Run options: include {:focus=>true}

All examples were filtered out; ignoring {:focus=>true}
[...]

Finished in 6.29 seconds (files took 37.34 seconds to load)
217 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