[Interpreter] Disable mjsunit/readonly test on ignition due to flakes.

BUG=v8:4280
LOG=N
NOTRY=True
TBR=machenbach@chromium.org

Review URL: https://codereview.chromium.org/1438463002

Cr-Commit-Position: refs/heads/master@{#31900}

test/mjsunit/mjsunit.status

@@ -794,6 +794,7 @@
   'osr-elements-kind': [SKIP],
   'property-load-across-eval': [SKIP],
   'proto-accessor': [SKIP],
+  'readonly': [SKIP],
   'regress-3225': [SKIP],
   'regress/clear-keyed-call': [SKIP],
   'regress/regress-102153': [SKIP],