[Fix] Move handler_test.go into internal/server dir

[Ujstor]

Tell us about your feature request

Common practice is to place test files in the same directory as the Go files they test. We can move handler_test.go into internal/server/routes_test.go and remove the tests directory from the root. Also, update the docs that refer to the old project tree.

This would be consistent with PR #271, which introduces the DB integration test.

Disclaimer

• I agree