CMake: Add target to build all tests

- Also ensure mc-tests is a target for ctest

Change-Id: Id2268b072d731ec69fbd45bde85238e8acfcd29f
Reviewed-on: http://review.couchbase.org/37387
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>