gdb: add tests for per-inferior settings

gdb: add tests for per-inferior settings

I am currently upstreaming some patches from ROCgdb that were made a few
years ago. This series cleans up how the CLI and MI access parameters
for multiple inferiors.

A preceding patch (702991711a91 ("gdb: Have setter and getter callbacks
for settings")) allowed for per-inferior getter/setter functions of some
inferior-specific settings. Over the years a few changes were accepted
upstream (cc09d372f664 ("gdb: make set/show args work with
$_gdb_setting_str")) that implemented those capabilities.

This patch adds a few tests to verify that those settings are properly
set and accessed from the CLI, MI as well as the python interface.

Co-Authored-By: Simon Marchi <simon.marchi@efficios.com>
Change-Id: I7a5eab210c84d6a6782a32125f68bde34d9a2339
Approved-By: Andrew Burgess <aburgess@redhat.com>