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

src/mm-context.h

@@ -46,5 +46,8 @@ gboolean     mm_context_get_log_relative_timestamps (void);
 gboolean     mm_context_get_test_session    (void);
 gboolean     mm_context_get_test_enable     (void);
 const gchar *mm_context_get_test_plugin_dir (void);
+#if defined WITH_UDEV
+gboolean     mm_context_get_test_no_udev    (void);
+#endif
 
 #endif /* MM_CONTEXT_H */