Merge #18267: BIP-325: Signet [consensus]

8258c4c007 test: some sanity checks for consensus logic (Anthony Towns)
e47ad375bf test: basic signet tests (Karl-Johan Alm)
4c189abdc4 test: add small signet fuzzer (practicalswift)
ec9b25d046 test: signet network selection tests (Karl-Johan Alm)
3efe298dcc signet: hard-coded parameters for Signet Global Network VI (2020-09-07) (Karl-Johan Alm)
c7898bca4e qt: update QT to support signet network (Karl-Johan Alm)
a8de47a1c9 consensus: add signet validation (Karl-Johan Alm)
e8990f1214 add signet chain and accompanying parameters (Karl-Johan Alm)
404682b7cd add signet basic support (signet.cpp) (Karl-Johan Alm)
a2147d7dad validation: move GetWitnessCommitmentIndex to consensus/validation (Karl-Johan Alm)

Pull request description:

  This PR is a part of BIP-325 (https://github.com/bitcoin/bips/blob/master/bip-0325.mediawiki), and is a sub-PR of #16411.

  * Signet consensus (this)
  * Signet RPC tools (pending)
  * Signet utility scripts (contrib/signet) (pending)

ACKs for top commit:
  jonatack:
    re-ACK 8258c4c007 per `git diff dbeea65 8258c4c`, only change since last review is updated `-signet*` config option naming.
  fjahr:
    re-ACK 8258c4c
  laanwj:
    ACK 8258c4c007
  MarcoFalke:
    Approach ACK 8258c4c007 🌵

Tree-SHA512: 5d158add96755910837feafa8214e13695b769a6aec3a2da753cf672618bef377fac43b0f4b772a87b25dd9f0c1c9b29f2789785d7a7d47a155cdcf48f7c975d

test/functional/test_runner.py

@@ -208,6 +208,7 @@ BASE_SCRIPTS = [
     'rpc_bind.py --ipv6',
     'rpc_bind.py --nonloopback',
     'mining_basic.py',
+    'feature_signet.py',
     'wallet_bumpfee.py',
     'wallet_implicitsegwit.py',
     'rpc_named_arguments.py',