build: add build of empty tests

Add tests to the build.

Change-Id: Ic12b5eee708a3ec8e16d997f2a7d18adec2c779d
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/Makefile.am b/Makefile.am
index 0bc2b36..d37ef87 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -21,3 +21,5 @@
 	$(PHOSPHOR_LOGGING_CFLAGS) \
 	$(LIBIPMID_CFLAGS) \
 	-flto
+
+SUBDIRS = . test