Switch testing framework from MAIN to new UNITTEST network

UNITTEST inherites from MAIN but allows synamically changing its parameters using the ModifiableParams() interface
Avoid introducing a virtual into CChainParams

Treat fSkipProofOfWorkCheck the same as other parameters.
Merge pull request #5030

fbd36d8 Avoid introducing a virtual into CChainParams (Wladimir J. van der Laan)
f0fd00c Switch testing framework from MAIN to new UNITTEST network (SergioDemianLerner)