[avalanche] Add a facility for managing the command line options in the processor tests

Summary:
Some tests use a local ArgsManager, some use the global gArgs, some clear them after the test and some don't.
Let's clean this up and remove some boilerplate by consistently using m_node.args, and provide a facility that ensure all args are cleared after each test case.

Depends on D13005.

Test Plan:

ninja check-avalanche

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13011