LP #1651121: Test innodb.percona_changed_page_bmp_debug unstable under load

**Reported in Launchpad by Laurynas Biveinis last update 28-12-2016 13:25:53

On 5.6 trunk, under Valgrind:

innodb.percona_changed_page_bmp_debug w4 [ fail ]
...
==14619== Command: /mnt/workspace/percona-server-5.6-valgrind/BUILD_TYPE/valgrind/Host/ubuntu-xenial-64bit/build/sql/mysqld-debug --defaults-group-suffix=.1 --defaults-file=/mnt/workspace/percona-server-5.6-valgrind/BUILD_TYPE/valgrind/Host/ubuntu-xenial-64bit/build/mysql-test/var/4/my.cnf --log-output=file --loose-debug-sync-timeout=6000 --loose-skip-log-bin --innodb_track_changed_pages=TRUE --innodb_log_file_size=5M --innodb-file-per-table -#d,bitmap_page_2_write_error --core-file
...
2016-12-18 00:43:01 17cfb700 InnoDB: Assertion failure in thread 399488768 in file log0online.cc line 1203
InnoDB: Failing assertion: bmp_tree_node
...

This is a regression from diagnostics added for bug 1625466: it is not guaranteed that the first bitmap write attempt will have two bitmap data pages, as it may happen in the system tablespace before the two two-page-creating workload.