MDEV-18663 Tests : use --core-file if mariabackup output is redirected to a file

This is done in order to help debugging buildbot failures.

mysql-test/suite/mariabackup/encrypted_page_compressed.test

@@ -36,7 +36,7 @@ echo # xtrabackup backup;
 let $targetdir=$MYSQLTEST_VARDIR/tmp/backup;
 let $backuplog=$MYSQLTEST_VARDIR/tmp/backup.log;
 --error 1
-exec $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf --backup --target-dir=$targetdir > $backuplog;
+exec $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf --backup --target-dir=$targetdir --core-file > $backuplog;
 --enable_result_log
 
 --let SEARCH_PATTERN=Database page corruption detected.*

mysql-test/suite/mariabackup/encrypted_page_corruption.test

@@ -51,7 +51,7 @@ let $backuplog=$MYSQLTEST_VARDIR/tmp/backup.log;
 
 --disable_result_log
 --error 1
-exec $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf --backup --extended-validation --target-dir=$targetdir > $backuplog;
+exec $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf --backup --extended-validation --target-dir=$targetdir --core-file > $backuplog;
 --enable_result_log
 
 

mysql-test/suite/mariabackup/log_checksum_mismatch.test

@@ -7,7 +7,7 @@ let $targetdir=$MYSQLTEST_VARDIR/tmp/backup;
 let $backuplog=$MYSQLTEST_VARDIR/tmp/backup.log;
 
 --disable_result_log
-exec  $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf --backup --target-dir=$targetdir --dbug=+d,log_intermittent_checksum_mismatch > $backuplog;
+exec  $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf --backup --target-dir=$targetdir --dbug=+d,log_intermittent_checksum_mismatch --core-file > $backuplog;
 --enable_result_log
 
 --let SEARCH_RANGE = 10000000

mysql-test/suite/mariabackup/unencrypted_page_compressed.test

@@ -38,7 +38,7 @@ echo # xtrabackup backup;
 let $targetdir=$MYSQLTEST_VARDIR/tmp/backup;
 let $backuplog=$MYSQLTEST_VARDIR/tmp/backup.log;
 --error 1
-exec $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf --backup --target-dir=$targetdir > $backuplog;
+exec $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf --backup --target-dir=$targetdir --core-file > $backuplog;
 --enable_result_log
 
 --let SEARCH_PATTERN=Database page corruption detected.*