test/signer_mock: return signatures of real size

Size of a signature affects the weight of transaction, which is verified
in tests.

Also method SignOutputRaw now returns the number of signatures matching
the number of signature descriptors to prevent crashes in tests where
multiple inputs are signed.

test/signer_mock.go

@@ -27,7 +27,12 @@ func (s *mockSigner) SignOutputRaw(ctx context.Context, tx *wire.MsgTx,
 		SignDescriptors: signDescriptors,
 	}
 
-	rawSigs := [][]byte{{1, 2, 3}}
+	rawSigs := make([][]byte, len(signDescriptors))
+	for i := range signDescriptors {
+		sig := make([]byte, 64)
+		sig[0] = byte(i + 1)
+		rawSigs[i] = sig
+	}
 
 	return rawSigs, nil
 }
@@ -118,7 +123,10 @@ func (s *mockSigner) MuSig2Sign(context.Context, [32]byte, [32]byte,
 func (s *mockSigner) MuSig2CombineSig(context.Context, [32]byte,
 	[][]byte) (bool, []byte, error) {
 
-	return true, nil, nil
+	sig := make([]byte, 64)
+	sig[0] = 42
+
+	return true, sig, nil
 }
 
 // MuSig2Cleanup removes a session from memory to free up resources.