Keep only one common file for serialized certificates

Issue from discussion #612 (closed)
Files:

• samples/embedded/cli_pubsub_server/src/static_security_data.h
• tests/ClientServer/validation_tests/server/static_security_data.h

These files are used to obtain the certificate when the target does not have a file system. The certificates are the same in both files, the only difference being the addition of user certificates in the test file. We shall keep only one common file.