[bitcoin-dev] Introduce N testnet chains to test different block sizes

["Jorge Timón" <jtimon@jtimon.cc>]

I have created the following PR that simplifies testing of different
block sizes and (if it were merged) would also slightly simplify a
future block size change hardfork.

https://github.com/bitcoin/bitcoin/pull/6382

I hope someone finds this useful. Please, post to github if you find
any issues. But, please, don't discuss the block size issue itself in
this post or the PR, the size is simply -blocksize.

I repeat the text here:

It would be generally good to have more people collecting data and
conduction simulations related to different consensus maximum block sizes.
This PR attempts to simplify that work.
Even if it may take long until it is merged (because it requires many
little steps to be taken first), this branch (or a fork of it) can be
used right now for
testing purposes.

One can use it, for example, like this: ```./src/qt/bitcoin-qt
-chain=sizetest -debug -printtoconsole -gen=1 -genproclimit=20
-blocksize=2000000```

I will rebase and update the list of dependencies accordingly as
things get merged.

Dependencies:

- Chainparams: Translations: DRY: options and error strings #6235
- CTestNetParams and CRegTestParams extend directly from CChainParams #6381