ClearArg also clears multi-args, add unit tests for args clearing

Summary:
ClearArg() did not clear the multi arg map, which could lead to the
situation where GetArg("arg") returned its default value while
GetArgs("arg") returned a non empty vector.
Add unit tests for the ClearArg() method.

Test Plan:

make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2290