[DBClientTest] Try to avoid the test error on TravisCI.

After commit: bfe133a3a, the test on TravisCI has failed with
the following message.

  testHatoholArmPluginGate::test_startAndWaitExit:	[ERR] <ChildProcessManager.cc:300> Failed to create process: (hapi-test-plugin), Failed to execute child process "hapi-test-plugin" (No such file or directory)
[ERR] <HatoholArmPluginGate.cc:247> Failed to execute: (104) hapi-test-plugin