[electrum] add test coverage for base58 address encoding

Summary:
Review of D16278 revealed that there is not test coverage for hash160_to_p2sh

Add a test vector for this function and two adjacent functions.

Cleanup TestBitcoin._do_test_crypto: remove a to call to public_key_to_p2pkh whose return value was ignored. We add coverage in the new test for this function.

Depends on D16277

Test Plan: python -m electrumabc.tests.test_bitcoin

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16303