ecash: make XEC/eCash the default

Summary:

• makes -ecash default to true, rather than false
• stops manual setting it to true in tests, since they were already expecting XEC results, except for:
• adapts the bitcoin-tx test vectors which I have overlooked in previous patches

after this, anyone wanting to use the old BCHA default unit and CashAddr prefixes is required to start the node with ecash=0

Test Plan:

ninja all check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien, Mengerian, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D9871