[REEF-1865] Allow runyarntests.sh run individual unit tests
[REEF-1865] Allow runyarntests.sh run individual unit tests

JIRA:
  [REEF-1865](https://issues.apache.org/jira/browse/REEF-1865)

Pull Request:
  Closes #1366

singlis/reef,dongjoon-hyun/reef,singlis/reef,shravanmn/reef,apache/reef,jwang98052/reef,singlis/reef,dougmsft/reef,apache/reef,markusweimer/reef,dougmsft/reef,dongjoon-hyun/incubator-reef,shravanmn/reef,markusweimer/incubator-reef,jwang98052/incubator-reef,jwang98052/incubator-reef,jwang98052/reef,singlis/reef,apache/incubator-reef,dongjoon-hyun/incubator-reef,apache/incubator-reef,markusweimer/incubator-reef,markusweimer/incubator-reef,markusweimer/reef,jwang98052/incubator-reef,jwang98052/incubator-reef,jwang98052/incubator-reef,apache/incubator-reef,apache/incubator-reef,dougmsft/reef,markusweimer/reef,apache/incubator-reef,jwang98052/reef,markusweimer/reef,markusweimer/incubator-reef,markusweimer/incubator-reef,apache/incubator-reef,dongjoon-hyun/reef,dongjoon-hyun/incubator-reef,apache/reef,motus/reef,jwang98052/incubator-reef,shravanmn/reef,dongjoon-hyun/reef,dongjoon-hyun/reef,motus/reef,motus/reef,apache/reef,dongjoon-hyun/reef,singlis/reef,motus/reef,singlis/reef,markusweimer/incubator-reef,motus/reef,dougmsft/reef,dongjoon-hyun/incubator-reef,dougmsft/reef,jwang98052/incubator-reef,dongjoon-hyun/incubator-reef,shravanmn/reef,markusweimer/reef,jwang98052/reef,dongjoon-hyun/reef,dongjoon-hyun/reef,shravanmn/reef,markusweimer/reef,dougmsft/reef,motus/reef,dongjoon-hyun/incubator-reef,dongjoon-hyun/incubator-reef,markusweimer/incubator-reef,apache/reef,motus/reef,singlis/reef,apache/reef,shravanmn/reef,jwang98052/reef,shravanmn/reef,apache/reef,markusweimer/reef,jwang98052/reef,apache/incubator-reef,jwang98052/reef,dougmsft/reef