Make a test more robust

Sometimes, the test would fail with a result difference for
the READ UNCOMMITTED read, because the incremental backup
would finish before redo log was written for all the rows
that were inserted in the second batch.

To fix that, cause a redo log write by creating another
transaction. The transaction rollback (which internally does commit)
will be flushed to the redo log, and before that, all the preceding
changes will be flushed to the redo log as well.

mysql-test/suite/mariabackup/apply-log-only-incr.result

@@ -3,6 +3,10 @@ CREATE TABLE t(a INT UNSIGNED PRIMARY KEY) ENGINE INNODB;
 INSERT INTO t VALUES(0);
 COMMIT;
 start transaction;
+BEGIN;
+DELETE FROM t LIMIT 1;
+SET GLOBAL innodb_flush_log_at_trx_commit = 1;
+ROLLBACK;
 NOT FOUND /Rollback of trx with id/ in current_test
 # expect NOT FOUND
 NOT FOUND /Rollback of trx with id/ in current_test

mysql-test/suite/mariabackup/apply-log-only-incr.test

@@ -26,6 +26,13 @@ eval INSERT t VALUES(201-$n);
 dec $n;
 }
 --enable_query_log
+connect (flush_log,localhost,root,,);
+BEGIN;
+DELETE FROM t LIMIT 1;
+SET GLOBAL innodb_flush_log_at_trx_commit = 1;
+ROLLBACK;
+disconnect flush_log;
+connection default;
 
 --disable_result_log
 exec $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf  --backup --ftwrl-wait-timeout=5 --ftwrl-wait-threshold=300 --ftwrl-wait-query-type=all --target-dir=$incremental_dir --incremental-basedir=$basedir ;