test: remove flaky status from eval_messages test

This test has not failed on armv7-wheezy in over 6 weeks. Let's remove its
"flaky" status.

Fixes: https://github.com/nodejs/node/issues/2672


Reviewed-By: Rod Vagg <r@va.gg>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/3420