MDEV-26870 --skip-symbolic-links does not disallow .isl file creation

The InnoDB DATA DIRECTORY attribute is not implemented via
symbolic links but something similar, *.isl files that contain
the names of data files.

InnoDB failed to ignore the DATA DIRECTORY attribute even though
the server was started with --skip-symbolic-links.

Native ALTER TABLE in InnoDB will retain the DATA DIRECTORY attribute
of the table, no matter if the table will be rebuilt or not.

Generic ALTER TABLE (with ALGORITHM=COPY) as well as TRUNCATE TABLE
will discard the DATA DIRECTORY attribute.

All tests have been run with and without the ./mtr option
--mysqld=--skip-symbolic-links
and some tests that use the InnoDB DATA DIRECTORY attribute
have been adjusted for this.

mysql-test/suite/mariabackup/partition_datadir.test

@@ -1,4 +1,5 @@
 --source include/have_partition.inc
+--source include/have_symlink.inc
 let $targetdir=$MYSQLTEST_VARDIR/backup;
 mkdir $targetdir;
 mkdir $MYSQLTEST_VARDIR/partitdata;