dm: test: Make use of CONFIG_UNIT_TEST

At present we always include test/dm from the main Makefile. We have a
CONFIG_UNIT_TEST that should control whether the test/ directory is built,
so rely on that instead.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/Makefile b/Makefile
index 5b80eb2..89c42c1 100644
--- a/Makefile
+++ b/Makefile
@@ -807,7 +807,7 @@
 ifdef CONFIG_POST
 libs-y += post/
 endif
-libs-$(CONFIG_UNIT_TEST) += test/ test/dm/
+libs-$(CONFIG_UNIT_TEST) += test/
 libs-$(CONFIG_UT_ENV) += test/env/
 libs-$(CONFIG_UT_OPTEE) += test/optee/
 libs-$(CONFIG_UT_OVERLAY) += test/overlay/