base-manager: allow forcing the testing without udev

Even if udev support is really built and available.

This is extremely useful to test the udev-less setup without fully recompiling the whole daemon.

E.g.: the daemon can be run like this:

    $ sudo /usr/sbin/ModemManager --debug --test-no-udev

And then, the kernel events may be reported using mmcli like this:

    $ sudo mmcli --report-kernel-event-auto-scan