dm: usb: Deal with USB keyboard persisting across tests

Clear any USB-keyboard devices before running a unit test, to avoid
using a stale udevice pointer in stdio. Add a long comment to explain
this situation and why this solution seems best, at least for now.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/console.h

@@ -179,6 +179,14 @@ void console_puts_select_stderr(bool serial_only, const char *s);
  */
 int console_clear(void);
 
+/**
+ * console_remove_by_name() - Remove a console by its stdio name
+ *
+ * This must only be used in tests. It removes any use of the named stdio device
+ * from the console tables.
+ */
+int console_remove_by_name(const char *name);
+
 /*
  * CONSOLE multiplexing.
  */