UNITTEST parameters are not used by any current tests, and the model (modifyable parameters) is inconvenient when unit-testing in random order. As they are stored in a global structure every test would have to (re)set up its own parameters. This reverts #5030.
For consistency it is better, too, to test with the actual MAIN parameters and not a variation.
Also removes related (and unused) fSkipProofOfWork parameter.