Generate a privkey in the test framework instead of using dumpprivkey so
that descriptor wallets work in this test.
This is a backport of core#18788 [4/16] & core#20322 (adds a `sync_all`)
https://github.com/bitcoin/bitcoin/pull/18788/commits/c2711e4230d9a423ead24f6609691fb338b1d26b
https://github.com/bitcoin/bitcoin/pull/20322/commits/444412821e5349ce0e0b039d884583edb70c4399
I also had to add a `key_to_p2pkh_cashaddr` function to get the address in the correct format. This has the added benefit of adding coverage for `cashaddr.encode[_full]` which was unused before this commit, as far as I can tell.