This occurs because the ShowFileRequestHandler isn't enabled by default in the multicore example configurations (showing the config fails in the same way).
I'm attaching a patch which changes the multicore example configurations to enable all admin handlers through AdminHandlers. The patch also removes some parameters from the <admin> configuration element which appear to be used only in deprecated stuff.
The patch furthermore changes index.jsp to check for the existence of the ShowFileRequestHandler or the all-in-one AdminHandlers before displaying the links to get schema and config. Also the deprecated getSchemaFile()/getConfigFile() methods are changed to getSchemaResource()/getConfigResource().
I've done a quick search for more solrconfig.xml with deprecated stuff in them and updated the ones in solr-ruby, but not the ones used in various tests. This should probably be a separate issue?