dm: test: usb: Add tests for the 'usb tree' command

Add tests that this command produces the right output, even when a rescan
results in a device disappearing from the bus.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/test.dts b/arch/sandbox/dts/test.dts
index e2c4971..52749c0 100644
--- a/arch/sandbox/dts/test.dts
+++ b/arch/sandbox/dts/test.dts
@@ -307,12 +307,24 @@
 				compatible = "sandbox,usb-hub";
 				#address-cells = <1>;
 				#size-cells = <0>;
-				flash-stick {
+				flash-stick@0 {
 					reg = <0>;
 					compatible = "sandbox,usb-flash";
 					sandbox,filepath = "testflash.bin";
 				};
 
+				flash-stick@1 {
+					reg = <1>;
+					compatible = "sandbox,usb-flash";
+					sandbox,filepath = "testflash1.bin";
+				};
+
+				flash-stick@2 {
+					reg = <2>;
+					compatible = "sandbox,usb-flash";
+					sandbox,filepath = "testflash2.bin";
+				};
+
 			};
 		};
 	};