Add unit tests for "validateExpirationDateInternal"
Add unit tests for "validateExpirationDateInternal"

They were copied and adjusted as needed from the tests for
"validateExpirationDate".

Signed-off-by: Daniel Calviño Sánchez <bd7629e975cc0e00ea7cda4ab6a120f2ee42ac1b@gmail.com>

nextcloud/server,nextcloud/server,nextcloud/server,nextcloud/server