[Qt] add payment request unit test for non matching networks

- verify that payment request network matches client network
- add static verifyNetwork() function to PaymentServer to be able to use
  the same validation code in GUI and unit-testing code

src/qt/paymentserver.h

@@ -91,6 +91,9 @@ public:
     // This is now public, because we use it in paymentservertests.cpp
     static bool readPaymentRequestFromFile(const QString& filename, PaymentRequestPlus& request);
 
+    // Verify that the payment request network matches the client network
+    static bool verifyNetwork(const payments::PaymentDetails& requestDetails);
+
 signals:
     // Fired when a valid payment request is received
     void receivedPaymentRequest(SendCoinsRecipient);