Add missing tests for db.service_* methods

There was no tests in test_db_api for service_* methods.

Add tests to ensure that all works. Also these tests allow us to refactor
work with session in these methods.

blueprint db-api-tests

Change-Id: I0c3b233e1290f08ea01a8c64eacb95af1eae6f24
Merge "Add missing tests for db.service_* methods"