Fix #15409. The QT wallet fail to open the configuration file on Mac, when these is no default application for `*.conf` files.

Here is a feasible way to solve this bug. When `QDesktopServices::openUrl` fails to open `file:///path/bitcoin.conf` with its default application, use `QProcess::startDetached` to run `open -t /path/bitcoin.conf` command instead, so as to open the configuration file with system's default text editor.

ninja all check check-functional