[eCash] add the XEC base unit and test it by default
Summary:
- adds the XEC unit which is selected by the '-xec' CLI option
- enables it by default for unit tests
- adapts these tests to account for 2 decimal places rather than eight
Test Plan:
ninja all check
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Subscribers: Fabien
Differential Revision: https://reviews.bitcoinabc.org/D9657