Bug#35590128 Wrong first-row cost for 'Append'

Post-push fix: Skip new test case when using Hypergraph, as cost
figures are different from those of the traditional optimizer.

Change-Id: Ic6edd2e9d8aa0444f69d2ab8f9baa0c52afb112f

Author: Jan Wedvik

mysql-test/t/union.test

@@ -2830,6 +2830,7 @@ CREATE TABLE t1 (
 INSERT INTO t1 VALUES (0,0), (1,1), (2,2);
 ANALYZE TABLE t1;
 
+--skip_if_hypergraph  # Other costs with Hypergraph.
 EXPLAIN FORMAT=TREE (SELECT b FROM t1 UNION SELECT b FROM t1)
 UNION ALL SELECT b FROM t1;