Moved the test for RetainedEval to reef-tests

Closes #971

Also, this does some mild refactorings of the retained evaluators code base towards #928
Merge pull request #973 from Microsoft-CISL/weimer_971

Moved the test for RetainedEval to reef-tests.

Tests passed locally and on YARN (Windows)
Merge branch 'master' of https://github.com/Microsoft-CISL/REEF into yingdac-restart-complete-handler