aws: Fix CI for AWS Client example

In some cases the thread receiving the packets can print its acknoledge
"MQTT - Packet received. ReceivedBytes=2" before the  main thread can
print "main - Waiting for packet ack". This breaks the CI. CI doesn't
check for this line anymore.

Signed-off-by: Hugo L'Hostis <hugo.lhostis@arm.com>

templates/examples/aws-client/test.log

-main - Initialising network
 main - Starting kernel
-main - Awaiting network connection
-main - Network connection enabled
-main - Initialising MQTT connection
-main - Establishing TLS connection
 AWSN - [a-zA-Z0-9\.-]+.amazonaws.com [0-9]+(?:\.[0-9]+){3}:[0-9]+ connecting...
 AWSN - Successful connection to the server
 AWSN - Performing the TLS handshake...
@@ -11,19 +6,13 @@ AWSN - Successfully completed the TLS handshake
 AWSN - Verifying certificate...
 AWSN - Certificate verification passed
 AWSN - Established TLS connection
-main - Setuping MQTT connection
 MQTT - Packet received. ReceivedBytes=2.
 MQTT - CONNACK session present bit not set.
 MQTT - Connection accepted.
 MQTT - Received MQTT CONNACK successfully from broker.
 MQTT - MQTT connection established with the broker.
-main - Starting MQTT receiving thread
-main - Publishing demo message to OpenIoTSDK/demo
-main - Waiting for packet ack
 MQTT - Packet received. ReceivedBytes=2.
 MQTT - Ack packet deserialized with result: MQTTSuccess.
 MQTT - State record updated. New state=MQTTPublishDone.
-main - Terminating MQTT receiving thread
-main - Closing MQTT session and closing TLS connection
 MQTT - Disconnected from the broker.
 main - Demo finished !