test_mqtt_disconnect in mqtt_pubsub fails

[vanti]

Describe the bug
The test test_mqtt_disconnect fails with an assertion when run on cc3220sf_launchxl:

*** Booting Zephyr OS build zephyr-v2.1.0-1280-g523ccb7ec350  ***
Running test suite mqtt_test
===================================================================
starting test - test_mqtt_connect
MQTT connect failed -111
[mqtt_evt_handler:150] MQTT_EVT_CONNACK: Connected!
PASS - test_mqtt_connect
===================================================================
starting test - test_mqtt_subscribe
PASS - test_mqtt_subscribe
===================================================================
starting test - test_mqtt_publish_short
[mqtt_evt_handler:221] items: 1 packet id: 11234
PASS - test_mqtt_publish_short
===================================================================
starting test - test_mqtt_publish_long
[mqtt_evt_handler:177] MQTT_EVT_PUBACK packet id: 29227
PASS - test_mqtt_publish_long
===================================================================
starting test - test_mqtt_unsubscribe
[mqtt_evt_handler:234] packet id: 58081
PASS - test_mqtt_unsubscribe
===================================================================
starting test - test_mqtt_disconnect
[mqtt_evt_handler:156] MQTT_EVT_DISCONNECT: disconnected 0

    Assertion failed at ../src/test_mqtt_pubsub.c:460: test_mqtt_disconnect: test_disconnect() == TC_PASS is false

FAIL - test_mqtt_disconnect
===================================================================
Test suite mqtt_test failed.
===================================================================
PROJECT EXECUTION FAILED

To Reproduce
Steps to reproduce the behavior:

1. cd zephyr/tests/net/lib/mqtt_pubsub
2. Update src/config.h to use a valid mqtt broker, e.g.
   #define SERVER_ADDR "137.135.83.217"
3. west build -b cc3220sf_launchxl -p
4. west flash
5. Hit reset button
6. See error

Expected behavior
I'd expect the test to pass.

Impact
Unable to run the test successfully

Environment (please complete the following information):

• OS: Ubuntu 18.04
• Toolchain : Zephyr SDK 0.11.0
• Commit SHA or Version used: 471ffbe