SubscriptionManagerFCPTest.fcpCall(): Fix ignoring reply messages

- Symptom was that unit tests failed because ReplyReceiver contained no
reply when they expected one (NoSuchElementException).
- The reason for the ReplyReceiver not receiving the replies was that
fcpCall() uses FCPPluginClient.sendSynchronous() which in opposite to
send() returns the reply instead of passing it to the message handler
(the ReplyReceive is the message handler here)
- The fix is to store the reply to the ReplyReceiver manually in
fcpCall()