MDEV-27065 fixup: disable tests under valgrind

The server behaves differently on the DATA/INDEX DIRECTORY clause
under valgrind because symlink is disabled under valgrind.

mysql-test/suite/parts/inc/part_alter_values.inc

+--source include/have_symlink.inc
+
 --echo #
 --echo # MDEV-14641 Incompatible key or row definition between the MariaDB .frm file and the information in the storage engine
 --echo #

mysql-test/suite/parts/t/partition_alter_myisam.test

 --source include/have_partition.inc
---source include/have_symlink.inc
 --let $engine=MyISAM
 --source inc/part_alter_values.inc