itest: actually mint second tranche into existing group

Turns out we didn't really set up the group minting request properly to
mint into an existing group. Before this change we'd simply mint a
completely new group, using the same external key.

Author: guggero

itest/mint_fund_seal_test.go

@@ -577,6 +577,9 @@ func testMintExternalGroupKeyChantools(t *harnessTest) {
 	mintReq2 := CopyRequest(issuableAssets[0])
 	mintReq2.Asset.Name = "itestbuxx-money-printer-brrr-tranche-2"
 	mintReq2.Asset.ExternalGroupKey = externalGroupKey
+	mintReq2.Asset.GroupedAsset = true
+	mintReq2.Asset.NewGroupedAsset = false
+	mintReq2.Asset.GroupKey = batchAssets[0].AssetGroup.TweakedGroupKey
 
 	assetReqs2 := []*mintrpc.MintAssetRequest{mintReq2}