Allow cmake to run tests under Valgrind

CTest (part of CMake) can run configured unit tests under Valgrind's
leak checker by specifying "-T Memcheck" to ctest. However for this to
work the CTest module needs to be included in CMakeLists.txt.

Change-Id: Ica7da0902977ca45247cf539f7ea6616316e31eb
Reviewed-on: http://review.couchbase.org/42823
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Chris Hillery <ceej@couchbase.com>