Add unit tests for the (Soft|Force)Set(Multi)?Arg methods

Summary:
These functions are for testing purpose only and had no unit test

Depends on D2290

Test Plan:

./src/test/test_bitcoin --run_test=util_tests

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2291