[build] Add system_image property to package()

Change-Id: Ia7581f220c2725318cb89761bd2a0fea08ca5f9b
diff --git a/BUILD.gn b/BUILD.gn
index 5dbdcb4..c6bd0d5 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -174,6 +174,7 @@
 
 package("re2_tests") {
   testonly = true
+  system_image = true
 
   deps = [
     ":re2_slow_tests",