Rework SchedulerTestUtils with testing classes to replace mockito usages

Mockito is used in SchedulerTestUtils to mock ExecutionVertex and Execution for testing. It fails to mock every getter so that other tests use it may encounter NPE issues, e.g. ExecutionVertex#getID().
Mockito is also discouraged to be used in Flink tests. So I'd propose to rework the utils with testing classes.