test: dm: core: Add test case for uclass driver's child_post_probe()

Add test case to cover uclass driver's child_post_probe() method.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/test/dm/test-fdt.c b/test/dm/test-fdt.c
index 3da384f..e43acb2 100644
--- a/test/dm/test-fdt.c
+++ b/test/dm/test-fdt.c
@@ -55,10 +55,13 @@
 
 	/*
 	 * If this device is on a bus, the uclass_flag will be set before
-	 * calling this function. This is used by
-	 * dm_test_bus_child_pre_probe_uclass().
+	 * calling this function. In the meantime the uclass_postp is
+	 * initlized to a value -1. These are used respectively by
+	 * dm_test_bus_child_pre_probe_uclass() and
+	 * dm_test_bus_child_post_probe_uclass().
 	 */
 	priv->uclass_total += priv->uclass_flag;
+	priv->uclass_postp = -1;
 
 	return 0;
 }