Refactor MBeanServer tests to use custom service URL port

[sbrannen]

Overview

Several tests in our test suite that interact with an MBeanServer fail sporadically due to a BindException, indicating that multiple tests are attempting to create an MBeanServer on the same local port simultaneously. See SPR-PUBM-113 for an example.

Specifically, most of our JMX tests use the default service URL service:jmx:jmxmp://localhost:9875.

Commit b9013ad helps to mitigate build failures for such conflicts, but that is effectively only a "band aid" solution that doesn't actually address the underlying problem.

Proposal

We should refactor such tests to use SocketUtils to find an available port, compose a custom JMX service URL using that port, and start an MBeanServer for the particular test using that port.