Fix tests that fail on GC stress tests.

test/cctest/test-strings.cc

Index: test/cctest/test-strings.cc
diff --git a/test/cctest/test-strings.cc b/test/cctest/test-strings.cc
index 22a830295a70e19bdf5cc5c1af2931f1b98d3a98..11af3384b95fa13cc14b9b042785136c019008ab 100644
--- a/test/cctest/test-strings.cc
+++ b/test/cctest/test-strings.cc
@@ -1083,7 +1083,9 @@ TEST(SliceFromCons) {
   CHECK(parent->IsFlat());
   CHECK(slice->IsSlicedString());
   CHECK_EQ(SlicedString::cast(*slice)->parent(),
-           ConsString::cast(*parent)->first());
+           // Parent could have been short-circuited.
+           parent->IsConsString() ? ConsString::cast(*parent)->first()
+                                  : *parent);
   CHECK(SlicedString::cast(*slice)->parent()->IsSeqString());
   CHECK(slice->IsFlat());
 }