read_buffer_size.sh test fix

[Vasily Nemkov]

The test read_buffer_size.sh fails on some Jenkins slaves with following error:

2017-11-07 04:55:03: run.sh: Inserting more data into table
2017-11-07 04:55:03: run.sh: Inserting 501 rows into incremental_sample.test...
ERROR 1062 (23000) at line 1: Duplicate entry '1000' for key 'PRIMARY'

For details please see:
http://jenkins.percona.com/job/percona-xtrabackup-2.3-param-medium/29/BUILD_TYPE=debug,Host=xb-centos6-32,xtrabackuptarget=galera55/testReport/junit/(root)/t_read_buffer_size/sh/

[Vasily Nemkov (vasily.nemkov)]

PR to 2.3: https://github.com/percona/percona-xtrabackup/pull/452

[Vasily Nemkov (vasily.nemkov)]

PR to 2.4: https://github.com/percona/percona-xtrabackup/pull/463

[Shahriyar Rzayev (rzayev-sehriyar)]

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PXB-513