A unit test should not use a fix port to run a jmx connector

Today,the following 3 tests using a fixed port to start a jmx RMI Connector Server, it is too fragile because the fixed port could be used by others at testing time. We should use a dynamic port selected by the JMX factory.

test/com/sun/management/DiagnosticCommandMBean/DcmdMBeanDoubleInvocationTest.java
test/com/sun/management/DiagnosticCommandMBean/DcmdMBeanInvocationTest.java
test/com/sun/management/DiagnosticCommandMBean/DcmdMBeanTest.java