itest: verify group virtual transaction can be derived from PSBT

Adds a test to ensure the new PSBT field introduced in the previous
commit can be used to derive a transaction identical to the group
virtual transaction.

itest/mint_fund_seal_test.go

@@ -618,6 +618,17 @@ func unmarshalPendingAssetGroup(t *testing.T,
 	virtualTx, err := taprpc.UnmarshalGroupVirtualTx(a.GroupVirtualTx)
 	require.NoError(t, err)
 
+	// Ensure that the group virtual tx is the same as the grouped virtual
+	// PSBT.
+	groupVmPsbt, err := psbt.NewFromRawBytes(
+		bytes.NewReader(a.GroupVirtualPsbt), false,
+	)
+	require.NoError(t, err)
+	require.NotNil(t, groupVmPsbt)
+
+	require.Equal(t, *groupVmPsbt.UnsignedTx, virtualTx.Tx)
+
+	// Unmarshal group key request.
 	require.NotNil(t, a.GroupKeyRequest)
 	keyReq, err := taprpc.UnmarshalGroupKeyRequest(a.GroupKeyRequest)
 	require.NoError(t, err)