Add test for IP blocker in API v3 github endpoint

What does this MR do?

In #324244 (comment 531747412) the question came up if the v3 endpoints are covered by the group IP blocker feature. This MR adds a test to one of the endpoints that should verify the feature also works in v3

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 there is no user facing change.
• [-] 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