itest: assert string outpoint in SupplyCommitChainData is consistent

Modify itest helper to verify that the string-encoded outpoint in
SupplyCommitChainData matches the expected value based on existing
fields.

Author: ffranr

itest/assertions.go

@@ -2842,5 +2842,10 @@ func WaitForSupplyCommit(t *testing.T, ctx context.Context,
 		Index: fetchResp.ChainData.TxOutIdx,
 	}
 
+	require.Equal(
+		t, supplyCommitOutpoint.String(),
+		fetchResp.ChainData.CommitOutpoint,
+	)
+
 	return fetchResp, supplyCommitOutpoint
 }