test: MiniWallet: add helper methods `{send,create}_self_transfer_multi`
test: MiniWallet: add helper methods `{send,create}_self_transfer_multi`

Github-Pull: #24637
Rebased-From: eb3c5c4ef2eeb1d37d729d4487ed067a24cf81c8

bitcoinknots/bitcoin,bitcoinknots/bitcoin,bitcoinknots/bitcoin,bitcoinknots/bitcoin,bitcoinknots/bitcoin