SV2 testnet4

[plebhash]

close #41

#41 (comment)

For now I suggest cherry-picking the testnet4 commits on top of the sv2 branch. Once testnet4 is merged, I'll rebase.

@Sjors would you be willing to reconsider?

SRI tests are broken on testnet3, and we are pointing people to github.com/Sjors/bitcoin on the official Getting Started tutorial

cc @stratospher

[Sjors]

I think it's fine. I'll try to merge this and bake some new binaries later this week.

In the mean time, you could point people to your branch here.

[Sjors]

Thanks, I cherry-picked the testnet4 commits in my sv2 branch: bitcoin#29432 (comment)