test: check difficulty adjustment using alternate mainnet

test/functional/data/README.md

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+# Various test vectors
+
+## mainnet_alt.json
+
+For easier testing the difficulty is maximally increased in the first (and only)
+regarget period, by producing blocks approximately 2 minutes apart.
+
+The alternate mainnet chain was generated as follows:
+- use faketime to set node clock to 2 minutes after genesis block
+- mine a block using a CPU miner such as https://github.com/pooler/cpuminer
+- restart node with a faketime 2 minutes later
+
+```sh
+for i in {1..2015}
+do
+ faketime "`date -d @"$(( 1231006505 + $i * 120 ))"  +'%Y-%m-%d %H:%M:%S'`" \
+ bitcoind -connect=0 -nocheckpoints -stopatheight=$i
+done
+```
+
+The CPU miner is kept running as follows:
+
+```sh
+./minerd --coinbase-addr 1NQpH6Nf8QtR2HphLRcvuVqfhXBXsiWn8r --no-stratum --algo sha256d --no-longpoll --scantime 3 --retry-pause 1
+```
+
+The payout address is derived from first BIP32 test vector master key:
+
+```
+pkh(xprv9s21ZrQH143K3QTDL4LXw2F7HEK3wJUD2nW2nRk4stbPy6cq3jPPqjiChkVvvNKmPGJxWUtg6LnF5kejMRNNU3TGtRBeJgk33yuGBxrMPHi/44h/0h/0h/<0;1>/*)#fkjtr0yn
+```
+
+It uses `pkh()` because `tr()` outputs at low heights are not spendable (`unexpected-witness`).
+
+This makes each block determinisic except for its timestamp and nonce, which
+are stored in `mainnet_alt.json` and used to reconstruct the chain without
+having to redo the proof-of-work.
+
+The timestamp was not kept constant because at difficulty 1 it's not sufficient
+to only grind the nonce. Grinding the extra_nonce or version field instead
+would have required additional (stratum) software. It would also make it more
+complicated to reconstruct the blocks in this test.