test: mark test-vm-syntax-error-stderr as flaky

Ref: https://github.com/nodejs/node/issues/2660

PR-URL: https://github.com/nodejs/node/pull/2662
Reviewed-By: orangemocha - Alexis Campailla <orangemocha@nodejs.org>

test/sequential/sequential.status

@@ -11,6 +11,7 @@ test-repl-persistent-history                    : PASS,FLAKY
 [$system==win32]
 
 [$system==linux]
+test-vm-syntax-error-stderr                     : PASS,FLAKY
 
 [$system==macos]