Introduce user defined MQTT keep alive value (#73)

`MQTT_KEEP_ALIVE_INTERVAL_SECONDS` macro value
depends on the responsiveness of the target
running the tests because some targets might
be faster or slower than others. Hence, it
would be useful to have a default value for
this macro (5) and the integration code can
provide other value if needed.

Signed-off-by: Ahmed Ismail <[email protected]

Signed-off-by: Ahmed Ismail <[email protected]
Co-authored-by: Ahmed Ismail <[email protected]>

Author: urutva

src/mqtt/mqtt_test.c

@@ -170,7 +170,9 @@
  * @brief Time interval in seconds at which an MQTT PINGREQ need to be sent to
  * broker.
  */
-#define MQTT_KEEP_ALIVE_INTERVAL_SECONDS    ( 5U )
+#ifndef MQTT_KEEP_ALIVE_INTERVAL_SECONDS
+    #define MQTT_KEEP_ALIVE_INTERVAL_SECONDS    ( 5U )
+#endif
 
 /**
  * @brief The MQTT message published in this example.