test-tool: Set up GSettings keyfile

test-tool: Set up GSettings keyfile

Even with a separate XDG_CONFIG_HOME, settings may still be picked
up from the session's dconf instance (although the tool is typically
run through dbus-run-session).

To really ensure that we control the settings, we can force a different
backend. The 'memory' backend would be enough to ensure default values,
however using the 'keyfile' backend means we can also inject non-default
values.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3986>