Use local instance of ArgsManager in getarg_tests

Summary:
This test clobbers gArgs for any tests run after it.
Let's stop doing that to allow us to pass activation time via the command line to test tests in the future (diff coming soon).

Test Plan: make check

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4462