dm: test: Add a Kconfig file

Add a file to control driver model test features.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/Kconfig b/Kconfig
index 09e2bbe..6112a77 100644
--- a/Kconfig
+++ b/Kconfig
@@ -166,3 +166,5 @@
 source "fs/Kconfig"
 
 source "lib/Kconfig"
+
+source "test/Kconfig"