[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/test/paymentrequestdata.h

@@ -337,3 +337,27 @@ ssymvca1S/1KeM3n8Ydi2fi1JUzAAr59xPvNJRUeqCLP9upHn5z7br3P12Oz9A20\
 5/4wL4ClPRPVnOHgij0bEg+y0tGESqmF1rfOfXDszlo2U92wCxS07kq79YAZJ1Zo\
 XYh860/Q4wvc7lfiTe+dXBzPKAKhMy91yETY\
 ";
+
+//
+// Contains a testnet paytoaddress, so payment request network doesn't match client network
+//
+const char* paymentrequest1_cert2_BASE64 =
+"\
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+AdJbL3JOJLs6Eyp5zrPbfI8faRttFAdONKDrJgIpuW1E3g==\
+";