tests/driver_xbee: add DEVELHELP to catch non-valid configs

tests/driver_xbee/Makefile

@@ -24,6 +24,11 @@ CFLAGS += -DXBEE_UART=$(XBEE_UART)
 # No need of big buffer for this test
 CFLAGS += -DGNRC_PKTBUF_SIZE=512
 
+# Comment this out to disable code in RIOT that does safety checking
+# which is not needed in a production environment but helps in the
+# development process:
+CFLAGS += -DDEVELHELP
+
 # add current directory to the include path. Putting it in CFLAGS will make
 # it go to the beginning, before the standard includes.
 # That way xbee_params.h get's included and auto configuration can pick it up.