Add test case for replacing turbofan code for debugging.

test/mjsunit/mjsunit.status

Index: test/mjsunit/mjsunit.status
diff --git a/test/mjsunit/mjsunit.status b/test/mjsunit/mjsunit.status
index 15f89d88296cb228ab1d3b36f10c9ef500428389..b965d0296660af9ad1efeea25be3a3be4a4e5b3c 100644
--- a/test/mjsunit/mjsunit.status
+++ b/test/mjsunit/mjsunit.status
@@ -81,6 +81,10 @@
   'compiler/osr-assert': [PASS, NO_VARIANTS],
   'regress/regress-2185-2': [PASS, NO_VARIANTS],
 
+  # Issue 3660: Replacing activated TurboFan frames by unoptimized code does
+  # not work, but we expect it to not crash.
+  'debug-step-turbofan': [PASS, FAIL],
+
   # Support for %GetFrameDetails is missing and requires checkpoints.
   'debug-evaluate-bool-constructor': [PASS, NO_VARIANTS],
   'debug-evaluate-const': [PASS, NO_VARIANTS],