Fix GUI startup crash reported by Rspigler in
https://github.com/bitcoin/bitcoin/issues/24457 that happens if
settings.json contains an integer value for any of the configuration
options which GUI settings can currently clash with (-dbcache, -par,
-spendzeroconfchange, -signer, -upnp, -natpmp, -listen, -server, -proxy,
-proxy, -onion, -onion, -lang, and -prune).
Fix is a one-line change in ArgsManager::GetArg.
Just add tests. No changes to application behavior. Tests will be
updated in the next commit changing & improving current behavior.
Include a Qt test for GUI startup crash reported by Rspigler in
https://github.com/bitcoin/bitcoin/issues/24457 caused by GetArg
behavior that happens if settings.json contains an integer value for any
of the configuration options which GUI settings can currently clash with
(-dbcache, -par, -spendzeroconfchange, -signer, -upnp, -natpmp, -listen,
-server, -proxy, -proxy, -onion, -onion, -lang, and -prune).
