Add test case for DatetimeServiceFactory

[terrymanu]

This issue is for WeOpen-Star

The steps of add test case of SPI factories:

1. Create or reuse a SPI fixture, name as XXXFixture in test folder and do not forget put them into META-INF/services;
2. Add test case for XXXFactory's newInstance, findInstance, getInstance, getInstances, getAllInstances and so on;
   3.Please reference ClusterPersistRepositoryFactoryTest or ScalingEntryFactoryTest as samples.