I didn't know this was a constant so I'll give a shot to find it and reference it.
yeah, it dates back to the fact that solr will still work with "pre-multicore" solr home dirs that are really just single SolrCore instance dirs – in which case it refers to the core as "collection1" ... the constant you want is CoreContainer.DEFAULT_DEFAULT_CORE_NAME (which this test code should have been using in the first place, but as long as we're touching it let's fix it)
Are there any existing docs about how to create tests with Solr? I found there was some discussion on the mailing list. I'm happy to take a first pass at this if you think it'd help.
As far as i know the only docs are the javadocs in the base classes and test harness – if you'd like to take a stab at adding useful content to solr/test-framework/src/java/overview.html that would be awesome! (but perhaps deal with thta in a separate jira/patch so it doesn't hold up this already useful fix?)