Refactor pkg/server/server_test.go

[kyasbal]

pkg/server/server_test.go has been playing important part of testing on KHI.
It sends requests and verify responses one by one, but the test code was born at the very beginning of this project. The list of test cases becomes very long and there is many adhoc codes.
Now it's a little time consuming to add a new test when we add a new API. We need to seek a new way of the manner to write these endpoint tests.

https://github.com/GoogleCloudPlatform/khi/blob/main/pkg/server/server_test.go#L230