Fixed bug in optimizer_costs.test

The test failed if one had disabled some engines during compilation

mysql-test/main/optimizer_costs.result

@@ -344,4 +344,4 @@ drop table t1;
 #
 # cleanup
 #
-Start_engines: 9  End_engines: 10
+"New cost structures: 1 (should be 1)"

mysql-test/main/optimizer_costs.test

@@ -184,4 +184,6 @@ drop table t1;
 --echo #
 
 let $end_engines=`select count(*) from information_schema.optimizer_costs`;
---echo Start_engines: $start_engines  End_engines: $end_engines
+let $diff=`select $end_engines - $start_engines`;
+--echo "New cost structures: $diff (should be 1)"
+