test: Only descend into test/ when CONFIG_UNIT_TEST is enabled

The contents of the test subdirectories only make sense when we have
CONFIG_UNIT_TEST set.  We will otherwise attempt to build code on for
example sandbox that needs CONFIG_UNIT_TEST otherwise and rather than
complicate the Makefiles simply leave them out when we can.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/Makefile b/Makefile
index 5683714..f69aa06 100644
--- a/Makefile
+++ b/Makefile
@@ -724,8 +724,7 @@
 libs-y += env/
 libs-$(CONFIG_API) += api/
 libs-$(CONFIG_HAS_POST) += post/
-libs-y += test/
-libs-y += test/dm/
+libs-$(CONFIG_UNIT_TEST) += test/ test/dm/
 libs-$(CONFIG_UT_ENV) += test/env/
 libs-$(CONFIG_UT_OVERLAY) += test/overlay/