Tests for basic SASL mech handling

This uses LCB_CNTL_FORCE_SASL_MECH to verify that we can use a given
SASL method. This shall be extended in the future to check against
things like SASL_CRAM_MD5 and the like when the mock supports it

Change-Id: Ie0df3e78f5ae1fe2eb14591048cffe3dfc11db05
Reviewed-on: http://review.couchbase.org/29689
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>