[serializer] add test case for unknown external references.

test/cctest/cctest.status

Index: test/cctest/cctest.status
diff --git a/test/cctest/cctest.status b/test/cctest/cctest.status
index 1052e90d9531be0a2d0ffeecbd00454b1e2cb18f..eac5b92d8b88428f24d5026189c70ab6c21bad98 100644
--- a/test/cctest/cctest.status
+++ b/test/cctest/cctest.status
@@ -29,7 +29,6 @@
 [ALWAYS, {
   # All tests prefixed with 'Bug' are expected to fail.
   'test-api/Bug*': [FAIL],
-  'test-serialize/Bug*': [FAIL],
 
   ##############################################################################
 
@@ -90,6 +89,9 @@
   # BUG(3742).
   'test-mark-compact/MarkCompactCollector': [PASS, ['arch==arm', NO_VARIANTS]],
 
+  # Test that serialization with unknown external reference fails.
+  'test-serialize/SnapshotCreatorUnknownExternalReferences': [FAIL],
+
   ############################################################################
   # Slow tests.
   'test-api/Threading1': [PASS, ['mode == debug', SLOW]],