LP #1618555: undo_tablespaces.sh failures

**Reported in Launchpad by Sergei Glushchenko last update 22-11-2016 16:26:44

Test undo_tablespaces.sh sometimes fails with error:

InnoDB: Rollback of non-prepared transactions completed
2016-08-30 16:33:35 0x7fd60bfff700 InnoDB: Assertion failure in thread 140557301053184 in file buf0flu.cc line 3406
InnoDB: Failing assertion: srv_get_active_thread_type() == SRV_NONE
InnoDB: We intentionally generate a memory trap.
InnoDB: Submit a detailed bug report to http://bugs.mysql.com.
InnoDB: If you get repeated assertion failures or crashes, even
InnoDB: immediately after the mysqld startup, there may be
InnoDB: corruption in the InnoDB tablespace. Please refer to
InnoDB: http://dev.mysql.com/doc/refman/5.7/en/forcing-innodb-recovery.html
InnoDB: about forcing recovery.
16:33:35 UTC - xtrabackup got signal 6 ;
This could be because you hit a bug or data is corrupted.
This error can also be caused by malfunctioning hardware.
Attempting to collect some information that could help diagnose the problem.
As this is a crash and something is definitely wrong, the information
collection process might fail.

Thread pointer: 0x0
Attempting backtrace. You can use the following information to find out
where mysqld died. If you see no messages after this, something went
terribly wrong...
stack_bottom = 0 thread_stack 0x10000
/mnt/workspace/percona-xtrabackup-2.4-param/BUILD_TYPE/debug/Host/centos7-64/xtrabackuptarget/innodb56/storage/innobase/xtrabackup/src/innobackupex(my_print_stacktrace+0x35)[0x151d2f9]
/mnt/workspace/percona-xtrabackup-2.4-param/BUILD_TYPE/debug/Host/centos7-64/xtrabackuptarget/innodb56/storage/innobase/xtrabackup/src/innobackupex(handle_fatal_signal+0x2ab)[0x124fc91]
/lib64/libpthread.so.0(+0xf100)[0x7fd62f57c100]
/lib64/libc.so.6(gsignal+0x37)[0x7fd62d8d35f7]
/lib64/libc.so.6(abort+0x148)[0x7fd62d8d4ce8]
/mnt/workspace/percona-xtrabackup-2.4-param/BUILD_TYPE/debug/Host/centos7-64/xtrabackuptarget/innodb56/storage/innobase/xtrabackup/src/innobackupex[0xf2ee8f]
/mnt/workspace/percona-xtrabackup-2.4-param/BUILD_TYPE/debug/Host/centos7-64/xtrabackuptarget/innodb56/storage/innobase/xtrabackup/src/innobackupex(buf_flush_page_cleaner_coordinator+0xe70)[0x1029296]
/lib64/libpthread.so.0(+0x7dc5)[0x7fd62f574dc5]
/lib64/libc.so.6(clone+0x6d)[0x7fd62d994ced]

Please report a bug at https://bugs.launchpad.net/percona-xtrabackup
2016-08-30 16:33:35: run.sh: ===> innobackupex failed with exit code 2
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