QPID-3396 Fixed the typo from sasl_mech to sasl_mechs. This test was
passing (0-10 code path) due to the broker selecting CRAM_MD5.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1453951 13f79535-47bb-0310-9956-ffa450edef68