[server][testDBClientHatohol] Avoid a crash of the test.

When we use SYSTEM_ID_USER, OperationPrivilege doesn't access the config
DB. So the following problem can be avoided.

=========
https://travis-ci.org/project-hatohol/hatohol/jobs/20510396

  testDBClientHatohol::test_constructorDataQueryContext:[ERR] <DBAgentMySQL.cc:481> Failed to connect to MySQL: hatohol: (error: 1044, Domain ID: 0x30) Access denied for user 'hatohol'@'localhost' to database 'hatohol'
terminate called after throwing an instance of 'HatoholException'
  what():  <DBAgentMySQL.cc:90> Failed to connect to MySQL: hatohol: Access denied for user 'hatohol'@'localhost' to database 'hatohol'
!
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Crash: testDBClientHatohol::test_constructorDataQueryContext
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: (0.032216)