Use a separate watchonly wallet in rpc_fundrawtransaction.py
Summary:
Import things into a separate watchonly wallet to work with descriptor
wallets.
This is a backport of core#18788 [2/16]
https://github.com/bitcoin/bitcoin/pull/18788/commits/dc81418fd01021070f3f66bab5fee1484456691a
Test Plan: test/functional/test_runner.py rpc_fundrawtransaction
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D10654