Rewrite dar/JMSClientTestCase

I'm assigning this issue to you Alessio because you're the test author.

dar/JMSClientTestCase test uses MBean server and checks MessageCount attribute
for detecting whether message have been delivered to MDB, or not.

However this approach causes transient failures against different AS versions.
For example when I run that test standalone against AS 5.1.0, I'm getting failure for the first time.
Other test executions pass without problems (i.e. 2-nd and other tries).

Please consider using file approach instead of checking MBean server attributes.
This will be more reliable approach IMHO.