Switch of the test framework from MAIN to REGTEST

For several reasons the current test framework does not allow to easily incorporate new unit tests that append specially crafted blocks to the blockchain using ProcessBlock().
In the pull request https://github.com/bitcoin/bitcoin/pull/4688 we described the reasons and proposed a solution.
While we think that is the optimum solution, @gmaxwell and @laanwj suggested not implementing the changes because many files not directly related to the testing framework were modified by the patch.
In this pull request we implement @gmaxwell suggestion that the test framework should run under the RegTest network parameters and not under the main network parameters. Allthough this seems to be a small change, it was not. First, the RegTest block solving probability was 1/2 which still requires "mining" during the test case validation in order to dynamically create a block, which adds unnecessary complications to simple testing code.
To overcome this problem, the RegTest difficulty was changed from 0x207fffff to 0x2100FFFF (in compact notation), which gives an approximate 1/2^16 probability not to solve a block. Happily there was no lucky test case.
Because the maximum possible target (0xff ... 0xff) cannot be multiplied by 4 without overflow, ComputeMinWork() had to be modified to always return ProofOfWorkLimit() for the RegTest.

Also we found that several test cases make explicit use of properties of the MainNet. Such test cases are:

alert_tests.cpp
rpc_wallet_tests.cpp
main_tests.cpp
key_tests.cpp
DoS_tests.cpp
Checkpoints_tests.cpp
base58_tests.cpp
bloom_tests.cpp
rpc_tests.cpp
miner_tests.cpp

All these test cases were added a temporary switch from the RegTest network to the Main network in order to leave them working. Re-writting all of them for the RegTest probably requires more than 40 hours of work.

The alert_tests.cpp test case is special, because it requires the alert private key to generate inputs to the test cases. We suggest to switch this test case to the RegNet and replace the current RegTest alert private key with a published key-pair, so everyone can generate more test cases and there is no opaque data.

As the 4688  pull request, we've:

a. fixed the bug in miner_tests.cpp which leaves in the memory pool invalid transactions (mempool.clear() missing).
b. Added 7 unit tests:

    ToCheckBlockUpgradeMajority (untested before)
    EnforceBlockUpgradeMajority (untested before)
    RejectBlockOutdatedMajority (untested before)
    "bad-cb-height"
    "bad-version"
    "time-too-old"
    "bad-txns-nonfinal"

NOTE: In the file rpc_wallet_tests.cpp, the TABs were removed and replaced with whitespace indentation. This is the standard in the rest of the code. A whitespace-ignoring compare will show no important differences.

Sergio Demian Lerner & Timo Hanke

Yes, I was expecting you tell me this. I will remove this line later. I just let it there to make clear that there is a problem with the multiplication code that must be addressed if difficulty is going to be zero.