lib-tests-app: Fix build issue (#62)

The `freertos-iot-libraries-tests` application
was mistakenly relying on another app's TF-M configuration file.
The other app (`aws-iot-example`) was removed along with its
configuration files.

The `freertos-iot-libraries-tests` application now uses its
own TF-M configuration file.

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