[electrum] add a functional test for a reorg situation

Summary: This tests an important feature: how the wallet handles changes in transaction history caused by funny business on-chain.

Test Plan: pytest electrumabc/tests/regtest/test_reorg.py

Reviewers: #bitcoin_abc, Fabien, bytesofman

Reviewed By: #bitcoin_abc, Fabien, bytesofman

Subscribers: Fabien, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14784