Add "#undef NDEBUG" to all tests.

This change makes sure that even if the tests are built in a
"release" configuration (with optimizations and NDEBUG turned on),
the assertions won't get compiled out of the tests themselves.

The C standard guarantees that the most recent inclusion of
<assert.h> is the one that counts, so it's important that the
"#undef NDEBUG/#include <assert.h>" come as the last thing in
the block of header files.

"testutil.hpp" includes <assert.h>, so I've left <assert.h> out
of any test that #includes "testutil.hpp", just for the sake of
brevity.

tests/test_connect_delay.cpp

@@ -18,15 +18,15 @@ You should have received a copy of the GNU Lesser General Public License
 along with this program. If not, see <http://www.gnu.org/licenses/>.
 */
 
-
-#include <assert.h>
-#include <cstdlib>
-#include <cstring>
-#include <iostream>
-#include <errno.h>
-#include <unistd.h>
-
 #include "../include/zmq.h"
+#include <errno.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <string>
+
+#undef NDEBUG
+#include <assert.h>
 
 static void *server (void *)
 {