SolrJmxReporterTest is currently the only solr test using (the problematic) SolrTestCaseJ4.getNextAvailablePort helper method in order to assign a RMI port for running a JMX server.
This test doesn't actually care that it's using RMI for JMX, it only cares that:
- an MBeanServer is initialized that it can query
- that MBeanServer can be specified in the plugin configuration for the SolrJmxReporter
We should cleanup the test initialization so that it no longer depends on RMI via the serviceUrl plugin configuration option, and instead just use the agentId to specify an MBeanServer bootstrapped by the test.