graph test: Unmount image via image.Unmount()

This helps us track the unmount

graph_test.go

@@ -152,7 +152,7 @@ func TestMount(t *testing.T) {
 	}
 	// FIXME: test for mount contents
 	defer func() {
-		if err := Unmount(rootfs); err != nil {
+		if err := image.Unmount(runtime, rootfs, "testing"); err != nil {
 			t.Error(err)
 		}
 	}()