Fix for bug #15605: 5.0.17 test case 'mysqltest' failure.

mysql-test/t/mysqltest.test:
  Fix for bug #15605: 5.0.17 test case 'mysqltest' failure.
  - use 'test -s' instead of 'cat' to get assured 1 (cat returns >0 in case of errors)

mysql-test/t/mysqltest.test

@@ -968,9 +968,9 @@ drop table t1;
 
 --error 1
 --exec $MYSQL_TEST --record -x $MYSQL_TEST_DIR/var/tmp/bug11731.sql -R $MYSQL_TEST_DIR/var/tmp/bug11731.out
-# The .out file should be empty, cat will fail!
+# The .out file should be empty
 --error 1
---exec cat $MYSQL_TEST_DIR/var/tmp/bug11731.out
+--exec test -s $MYSQL_TEST_DIR/var/tmp/bug11731.out
 drop table t1;