MDEV-16571 - some backup tests sometimes with missing data after restore.

Marko mentions, it could be caused by  MDEV-15740 where InnoDB does not
flush redo log as often as it should, with innodb_flush_log_at_trx_commit=1

The workaround is to use innodb_flush_log_at_trx_commit=2, which,
according to MDEV-15740 is more durable.

mysql-test/suite/mariabackup/suite.opt

---innodb --loose-changed_page_bitmaps --innodb-sys-tables
+--innodb --loose-changed_page_bitmaps --innodb-sys-tables --innodb-flush-log-at-trx-commit=2

mysql-test/suite/mariabackup/unsupported_redo.result

@@ -5,9 +5,6 @@ call mtr.add_suppression("InnoDB: If you are installing InnoDB, remember that yo
 call mtr.add_suppression("InnoDB: Ignoring tablespace for `test`\\.`t21` because it could not be opened");
 call mtr.add_suppression("InnoDB: Cannot open datafile for read-only: ");
 call mtr.add_suppression("Table .* in the InnoDB data dictionary has tablespace id .*, but tablespace with that id or name does not exist");
-SELECT @@GLOBAL.innodb_flush_log_at_trx_commit;
-@@GLOBAL.innodb_flush_log_at_trx_commit
-1
 CREATE TABLE t1(i INT PRIMARY KEY auto_increment, a int) ENGINE INNODB;
 ALTER TABLE t1 FORCE, ALGORITHM=INPLACE;
 # Fails during full backup

mysql-test/suite/mariabackup/unsupported_redo.test

@@ -10,7 +10,6 @@ call mtr.add_suppression("Table .* in the InnoDB data dictionary has tablespace
 let $basedir=$MYSQLTEST_VARDIR/tmp/backup;
 let $incremental_dir=$MYSQLTEST_VARDIR/tmp/backup_inc1;
 
-SELECT @@GLOBAL.innodb_flush_log_at_trx_commit;
 CREATE TABLE t1(i INT PRIMARY KEY auto_increment, a int) ENGINE INNODB;
 --source ../../suite/innodb/include/no_checkpoint_start.inc
 ALTER TABLE t1 FORCE, ALGORITHM=INPLACE;