[Qt] add BIP70 DoS protection test

- this test required to make readPaymentRequestFromFile() public in order
  to be able to is it in paymentservertests.cpp

src/qt/test/paymentservertests.cpp

@@ -7,6 +7,7 @@
 #include "optionsmodel.h"
 #include "paymentrequestdata.h"
 
+#include "random.h"
 #include "util.h"
 #include "utilstrencodings.h"
 
@@ -108,6 +109,17 @@ void PaymentServerTests::paymentServerTests()
     r.paymentRequest.getMerchant(caStore, merchant);
     QCOMPARE(merchant, QString(""));
 
+    // Just get some random data big enough to trigger BIP70 DoS protection
+    unsigned char randData[BIP70_MAX_PAYMENTREQUEST_SIZE + 1];
+    GetRandBytes(randData, sizeof(randData));
+    // Write data to a temp file:
+    QTemporaryFile tempFile;
+    tempFile.open();
+    tempFile.write((const char*)randData, sizeof(randData));
+    tempFile.close();
+    // Trigger BIP70 DoS protection
+    QCOMPARE(PaymentServer::readPaymentRequestFromFile(tempFile.fileName(), r.paymentRequest), false);
+
     delete server;
 }