Move unittest constants to header

Allows unittests to easly access these constants w/o having to declare
extern const char* bla;

Test: th
Change-Id: I0bf6637430104c3d621277a725721ca61c6ac544
diff --git a/common/test_utils.h b/common/test_utils.h
index bb5a678..631ded4 100644
--- a/common/test_utils.h
+++ b/common/test_utils.h
@@ -33,6 +33,7 @@
 #include <gtest/gtest.h>
 
 #include "update_engine/common/action.h"
+#include "update_engine/common/testing_constants.h"
 #include "update_engine/common/utils.h"
 #include "update_engine/update_metadata.pb.h"