[FLINK-16300] Rework SchedulerTestUtils with testing classes to replace mockito usages

[zhuzhurk]

What is the purpose of the change

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.

Brief change log

Reworked the SchedulerTestUtils methods with testing classes.

Verifying this change

This change is already covered by existing tests

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Documentation

• Does this pull request introduce a new feature? (yes / no)
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Last check on commit d8c5f1c (Thu Feb 27 07:59:22 UTC 2020)

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[tillrohrmann]

Thanks for creating this PR @zhuzhurk. LGTM. +1 for merging.

[zhuzhurk]

Thanks for the reviewing @tillrohrmann .
Rebased the changes onto latest master and will merge it once travis gives green.