[electrum] add BIP44 mnemonic-to-address test with the eCash derivation paths

Summary:
While looking for test vectors for eCash address generation from seed, I noticed that Electrum ABC does not have a test using the m/44'/899'/0' derivation path.
It is useful to have such a test so we can share it with other wallets willing to support eCash and ensure everyone generates the same addresses from a given mnemonic phrase.

Add also tests for the 1899' path to check CashTab generates the same address. One of the tests matches an existing CashTab unittest, and I checked the other one by importing the seed into CashTab and comparing the first receive address.

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18424