[eCash] adapt functional tests to the XEC base unit and test it by default

Summary: apart from a regenerated rpc_getblockstats.json file, we basically multiply every 8 digit amount by a million.

Test Plan:

ninja all check
./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: Fabien, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9663