MDEV-14188 mariabackup.incremental_encrypted wrong result

Add an explicit redo log flush. In this test
innodb_flush_log_at_trx_commit was 2 by default.
It is also possible that this failure occurs because of MDEV-15740.

mysql-test/suite/mariabackup/incremental_encrypted.result

@@ -2,6 +2,7 @@ call mtr.add_suppression("InnoDB: New log files created");
 CREATE TABLE t(i INT) ENGINE INNODB ENCRYPTED=YES;
 INSERT INTO t VALUES(1);
 # Create full backup , modify table, then create incremental/differential backup
+SET GLOBAL innodb_flush_log_at_trx_commit = 1;
 INSERT INTO t VALUES(2);
 SELECT * FROM t;
 i

mysql-test/suite/mariabackup/incremental_encrypted.test

@@ -20,6 +20,7 @@ echo # Create full backup , modify table, then create incremental/differential b
 exec $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf  --backup --target-dir=$basedir;
 --enable_result_log
 
+SET GLOBAL innodb_flush_log_at_trx_commit = 1;
 INSERT INTO t VALUES(2);
 SELECT * FROM t;