Fix unittest key path in Brillo.

The unittests weren't able to find the keys if it's not run from the
update_engine_unittests directory.

Test: /data/nativetest/update_engine_unittests/update_engine_unittests
Bug: 26955860

Change-Id: I4c189cca2714986d3d2b2669b54a5dff9a488964
diff --git a/common/test_utils.cc b/common/test_utils.cc
index 2de8ced..13ce6f9 100644
--- a/common/test_utils.cc
+++ b/common/test_utils.cc
@@ -263,5 +263,9 @@
   return exe_path.DirName();
 }
 
+string GetBuildArtifactsPath(const string& relative_path) {
+  return GetBuildArtifactsPath().Append(relative_path).value();
+}
+
 }  // namespace test_utils
 }  // namespace chromeos_update_engine