[System.ServiceModel] Speed up Bug652331_2Test test execution.

[System.ServiceModel] Speed up Bug652331_2Test test execution.

For reasons I currently can't explain, the Service1Client test type
appears to support having only one async operation "in flight" at a
time. As a result of this, calling multiple Async methods on the
Service1Client instance, as was done previously, would result in not
all of the event handlers being invoked, and the test timing out
after 60s instead of exiting quickly because all event handlers had
been raised.

I don't understand enough of WCF & this code to say why Service1Client
can have only one async operation in progress at once, but making this
change so that each async operation operates on a separate instance
speeds up the Bug652331_2Test execution from 60s to 5s.