test: fix generation of conflicting transactions in chronik_electrum_blockchain.py

Summary:
I missed this in D17933. Apply the same solution as D17902: to make a conflicting tx now that MiniWallet is deterministic, we can specify a different target_size.

Also do some minor refactoring to avoid an extra call to self.wallet.create_self_transfer and pad_tx

Test Plan: with chronik compiled, ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17939