Refactor entity_leave_service_spec to make it more testable

What does this MR do?

Refactor the specs in entity_leave_service_spec to make it more testable. Lays the ground work for another MR to add additional specs.

I moved to using a combination of share examples and parameterized specs to make it easier to see what conditions are actually being tested. Particularly with combinations of group and project access levels, it was difficult to easily see what is covered and what is not. It DRYs up a lot of code.

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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

Closes #250689 (closed)