ReqMgr2 unittests

-unittests framework for WMCore.REST based application
-first unittest for ReqMgr2 example REST API

@ticoann please review, I have eventually made some clean up and reshuffling.
Merge pull request #4697 from zdenekmaxa/reqmgr2-unittest-for-newrestapi

ReqMgr2 unittests