Improve spec performance of EE::AuditEventService

What does this MR do?

Makes heavier use of let_it_be to avoid repetitive database calls to improve the run time of this spec from around 8 seconds to more like 1.5 seconds when run on a development environment.

Before:

Total events: 3779

Total time: 9.9845s

After:

Total events: 817

Total time: 1.0909s

Inferences:

Queries saved: 2962

Total Time taken decreased by 89.07%

Screenshots

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