tcmalloc: Only include tcmalloc tests if active allocator

Change the CMake configuration to only build the tcmalloc tests if we
are actually *using* tcmalloc, and not just if it was found (but a
different allocator was selected).

Change-Id: Icf40bf9b7dc6801ae274c868f19f52091b6dce22
Reviewed-on: http://review.couchbase.org/38542
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>