integration-tests: Update test configuration

* Change `ECHO_SERVER_ENDPOINT` from `localhost` to `127.0.0.1` as DNS
resolution of `localhost` fails with FreeRTOS-TCP/IP. Therefore
hardcode the numerical value.
* Increase the transport interface test timeout to its maximum value by
overriding the `TRANSPORT_TEST_WAIT_THREAD_TIMEOUT_MS` macro with value
`0xFFFFFFFF`. This is required to ensure the nightly test in the
internal CI passes.
* Increase MQTT process loop timeout, so that the expected packet from
the AWS IoT service is received successfully.

Signed-off-by: Devaraj Ranganna <devaraj.ranganna@arm.com>