Add more debug output for flaky debug-references test.

TBR=vogelheim@chromium.org

Review URL: https://codereview.chromium.org/1013603002

Cr-Commit-Position: refs/heads/master@{#27203}

test/mjsunit/debug-references.js

@@ -26,6 +26,8 @@
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 // Flags: --expose-debug-as debug --turbo-deoptimization
+// Flags: --stack-trace-on-illegal
+
 // Get the Debug object exposed from the debug context global object.
 Debug = debug.Debug