CMake: Allow to disable mock tests individually

This lets systems with more limited resources execute tests while
skipping mock tests, as an alternative to -DLCB_NO_TESTS=1

Change-Id: I38c09627b5b48188dfd2cece142c459c1a92bdf5
Reviewed-on: http://review.couchbase.org/43091
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>