Fix merging of global unknown data in PSBTs

Actually merge the global unknown key-value pairs.

Add a test for merging unknown key-value pairs.

src/script/sign.cpp

@@ -481,6 +481,7 @@ void PartiallySignedTransaction::Merge(const PartiallySignedTransaction& psbt)
     for (unsigned int i = 0; i < outputs.size(); ++i) {
         outputs[i].Merge(psbt.outputs[i]);
     }
+    unknown.insert(psbt.unknown.begin(), psbt.unknown.end());
 }
 
 bool PartiallySignedTransaction::IsSane() const