apps: Extract FreeRTOS IoT Libraries Tests app from aws-iot-example

The `aws-iot-example-tests` executable's source code and
CMake build instructions were contained within the AWS IoT Example
application. It has now been decoupled as a standalone application
named `freertos-iot-libraries-tests`.

The `freertos-iot-libraries-tests` application verifies this FRI's
integration of FreeRTOS IoT libraries running on the supported
hardware platforms for robustness and compatibility with AWS’s
published best practices for AWS IoT Core connectivity.

Other changes include:
* Use `CMAKE_PROJECT_NAME` to refer to the apps executable
* Include header files that declares objects used in source files

Signed-off-by: Hugues Kamba-Mpiana <hugues.kambampiana@arm.com>