Switch testing framework from MAIN to new UNITTEST network

UNITTEST inherites from MAIN but allows synamically changing its parameters using the ModifiableParams() interface
Suggested corrections on comments, variable names.
Also new test case testing the PoW skip in UNITTEST.
Avoid introducing a virtual into CChainParams

Treat fSkipProofOfWorkCheck the same as other parameters.
Merge pull request #4845

4705902 Avoid introducing a virtual into CChainParams (Wladimir J. van der Laan)
5e2e7fc Suggested corrections on comments, variable names. Also new test case testing the PoW skip in UNITTEST. (SergioDemianLerner)
a25fd6b Switch testing framework from MAIN to new UNITTEST network (SergioDemianLerner)