Add update_engine_unittests to the whitelist.

Test: ./site_utils/test_droid.py brillo_WhitelistedGtests
Bug: 28319454

Change-Id: Ia35f423eb86e1a472cbfd3e3a7243ce07b076f28
diff --git a/tests.txt b/tests.txt
index 7b7a598..2bd5101 100644
--- a/tests.txt
+++ b/tests.txt
@@ -20,4 +20,5 @@
 seccomp_bpf_test, no,
 shill_test, no,
 syscall_filter_unittest, no,
+update_engine_unittests, yes,
 weaved_test, no,