Add util_ArgParsing test

Summary:

ArgsManager test coverage for parsing of integer and boolean values is
currently very poor and doesn't give us a way of knowing whether changes to
ArgsManager may unintentionally break backwards compatibility, so this adds a
new test to catch regressions.

This is a backport of Core PR17390

Test Plan: ninja && ninja check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8259