test: mark eval_messages as flaky

This test has failed recently during a PR test in Jenkins, for
reasons seemingly not related to the change in the PR.

PR-URL: https://github.com/nodejs/node/pull/2648
Reviewed-By: evanlucas - Evan Lucas <evanlucas@me.com>

test/message/message.status

@@ -9,6 +9,7 @@ prefix message
 [$system==win32]
 
 [$system==linux]
+eval_messages                      : PASS,FLAKY
 
 [$system==macos]