MB-46000: Use cb_add_test_executable() for all test programs (re-apply)
MB-46000: Use cb_add_test_executable() for all test programs (re-apply)

Make use of the new cb_add_test_executable() CMake function to declare
targets which are test programs and not needed for a 'production'
build.

(Re-apply after previous revert, as 7.0.1 builds were still using
branch master and hence incorrectly picked this change up).

Change-Id: I05bb75979feb7f4729dab0b5572bed8c9a8785f1
Reviewed-on: http://review.couchbase.org/c/sigar/+/155802
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@couchbase.com>

couchbase/sigar,couchbase/sigar