Allow multiwallet.py to be used with --usecli

Summary:
Add test coverage for bitcoin-cli multiwallet calls.

Partial backport of core PR11970 (commit a14dbff)

Depends on D2399 and D2402

Test Plan:

./test/function/test_runner.py

Ensure that wallet_multiwallet.py is run twice, the second time with
the --usecli argument

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2400