[test] Mark slow test.

TBR=jkummerow@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/1807883002

Cr-Commit-Position: refs/heads/master@{#34815}

test/mozilla/mozilla.status

@@ -168,6 +168,8 @@
   # in the test: "This test will probably run out of memory".
   'js1_5/extensions/regress-345967': [SKIP],
 
+  # Slow with arm64 simulator in debug.
+  'ecma_3/Statements/regress-302439': [PASS, ['mode == debug', SLOW]],
 
   ##################### FLAKY TESTS #####################