Prevent TestNodeCLI.args mixups

Summary:
Change TestNodeCLI.call() to return a new instance instead of
modifying the existing instance. This way, it's possible to create
different cli objects that have their own options (for example
-rpcwallet options to connect to different wallets), and options set for
a single call (node.cli(options).method(args)) will no longer leak
into future calls.

Partial backport of core PR11970 (commit ca9085a)

Depends on D2396

Test Plan:

./test/functional/test_runner.py interface_bitcoin_cli

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2397