freertos-libs-test: Fix OTA PAL app signing key specification

For the integration test app, select the code signing public key .Pem
file within test directory of `FreeRTOS-Libraries-Integration-Tests`
for generating the credentials header file.

This static public key is necessary to verify the test signature
using the same key pair used to generate the signature during
provisioning.

Signed-off-by: Ahmed Ismail <Ahmed.Ismail@arm.com>