Most of our (~integration) tests based on testplanet runner.
However running testplanet for each test make the testing process slow.
It seems to be better to use real unit tests (without db dependency) when it's possible.
This patch makes small modification to make it possible to test orders.Service with real unit test.
As the existing unit test of `service.go` is isolated with `_test` package name, it's moved to an `_integration_test.go` file to make place for the unit test.
Change-Id: Ia69f26a34e2c48d230d8d36c2040dd02a60455a6
