I'm arguing AbstractSolrTestCase should be removed as it's obsoleted by SolrTestCaseJ4.
SOLR-3911 (back in 2012) Mark made it extend from SolrTestCaseJ4. There is really very little in this test class. Some of the methods here are duplicated by SolrTestCaseJ4 and thus are completely redundant (ignoreException, resetExceptionIgnores, getFile). There haven't been any modifications to this class of substance since 2012 either.
I think we can just outright remove it (no deprecation phase). Anyone still using it can trivially switch.